As of 1492 DR, who are the Masked Lords of Waterdeep?
The ones that I know of are:
Mirt, Jelenn Urmbrusk, Corylus Thann, Thardouk Starbuckler, and Dorgar Adarbrent (listed in Waterdeep: Dragon Heist).
The Forgotten Realms wiki lists a number of the Lords of Waterdeep as of 1491 DR, both living and dead. In addition to the Open Lord, Laeral Silverhand, it lists those Masked Lords described in Ed Greenwood's 2016 novel Death Masks, those mentioned in Waterdeep: Dragon Heist, and 2 others.
I haven't read Death Masks and don't know how significant the identities of the Masked Lords or their fates are to that story (i.e. how much of a spoiler each is), so I'll spoiler-tag each section as appropriate.
The premise of the novel, set in 1491 DR, is that some of the Masked Lords of Waterdeep are assassinated, and the culprits need to be discovered. (There's a preview of the 1st chapter linked from the product listing on WotC's website.) Naturally, the book provides the names of several Masked Lords that end up dead by the end; I'll note them later in the answer, as that list doesn't directly answer your question.
This post on Candlekeep Forum also discusses the events of Death Masks, and attempts to determine which characters are still Masked Lords by the end. There's obviously a lot of overlap with the lists from the wiki, but I did notice a few discrepancies between the two sources. I'll note the distinctions in brackets where relevant.
The Masked Lords who were not killed in the Death Masks assassinations and managed to keep their identities a secret from the public are:
- Omin Dran, half-elf male owner of Acquisitions Incorporated
- Mirt, human male former adventurer and moneylender (also appears in Waterdeep: Dragon Heist)
- 3 unnamed male lords whom Laeral called the Neutral Lords
Several other Masked Lords are unmasked in the course of the story:
- Gruthgar Hrimmrel, human male landlord and retired shipwright
- Lammakh Heirlarpost, human male businessman
- Kassalra Maremthur, human female ointment-maker
- Sarathue Serendragon, half-elf female [forum post spells her first name "Sarathlue", then "Sarathra" in a different line]
- Zereth Keltaerond, human male trader [forum post lists him under "New lords voted in that are seen as Lords before book's end"]
- Halark Tarncrown, human male merchant and investor [forum post also lists him under the same category as above]
According to the wiki, their identities were compromised when they were forced to attend a "volatile Lordsmoot" while unmasked.
[The forum post lists one more under the same category as Zereth and Halark:
- Perengal Yuskalaunt, shipping fleet owner]
It is unknown whether they remain Lords now that their identities are public.
A few Lords of Waterdeep were voted in immediately to replace those assassinated:
- Daerrask Querreth, human male investor [not listed in forum post]
- Zuzeena Qeldur, human female owner of a massive collection of small shops [forum post lists her under "New Lords voted in that are not known to have definitely accepted the role"]
- Cadraethe Haulhenarr, human female entrepreneur [same as above]
- Vaelra Kallo, human female dressmaker of North Ward [same as above]
Regarding the process by which Masked Lords are chosen/replaced, Forgotten Realms creator Ed Greenwood explains it in this Twitter thread from 2017:
Existing Masked Lords must vote you in. Open Lord knows all identities and controls slate [...] of candidates to possibly become Lords. [...] Accepting a Lordship (Masked Lord) is always voluntary, but sometimes life applies intense pressure to accept or decline.
According to the Candlekeep Forum post, Death Masks mentions a few other Masked Lord candidates that are proposed but not mentioned as being voted in:
One candidate is proposed by the half-elf Serendragon (her first name is spelled "Sarathra" in this line):
- Daranthra Xathnout, oxen-tender from Trades Ward [the FR wiki page mentions this in the line where Serendragon is listed]
The other three candidates are proposed by the 3 unnamed male "Neutral Lords":
- Belmark Chelvurr of Castle Ward, who maintains the hiring registry for caravan outriders and guards for mercantile companies in the city
- Emmura Flanthyn, owner of Flanthyn's Fastwares in South Ward, which delivered fresh groceries and other items to shops all over the city
- Ildunstran Wurth of Dock Ward, who dealt in fishmeal, night soil, and fertilizers, and transporting bulk root vegetables from nearby regions to Waterdeep
The forum post also notes the varying numbers of Masked Lords at various points throughout 1491 DR (as reported by characters in the book). It also notes some discrepancies in the numbers and ambiguity about who's being counted.
As the question notes, the Waterdeep: Dragon Heist adventure explicitly lists just a few of the Masked Lords (p. 160-161):
Mirt, Corylus Thann, Thardouk Starbuckler, Jelenn Urmbrusk, Dorgar Adarbrent
A certain character is referenced in the context of the Masked Lords in Waterdeep: Dungeon of the Mad Mage (p. 242):
It says that if the characters destroy the vampire on level 18, they receive the deed to an estate in Waterdeep with an official property transfer notice authorized by the Lords of Waterdeep. It's accompanied by a letter thanking them for destroying the vampire, signed with the initials "A.M."
If characters try to figure out who A.M. is, "Several Masked Lords of Waterdeep use every resource at their disposal to discourage and thwart investigations," but they might eventually discover who it was: "Artor Morlin, the Baron of Blood, a powerful vampire who dwells in a dungeon complex under Waterdeep and can't abide other vampires preying on his city. The baron's presence is one of Waterdeep's most closely guarded secrets."
The fact that several Masked Lords interfere with investigations into his identity, and that his presence is a "closely guarded secret" of the city, suggests that this character might be a Masked Lord himself.
This character's status as a Masked Lord is confirmed throughout the Adventurers League Season 8 modules:
Artor Morlin appears or is referenced in all 3 modules of the Umbral Aristocracy trilogy (DDAL08-01, 08-02, and 08-03). His status as a "masked lord" (lowercase) is mentioned in passing in the first two, but he plays a bigger role in DDAL08-03 "Dock Ward Double Cross", where he's referenced as a "masked lord of Waterdeep", a "masked lord", and a "Masked Lord" (uppercase).
Morlin reappears in the Folded Time trilogy (DDAL08-04, 08-05, and 08-06), in which characters are projected back in time. His status as a (future) "Masked Lord" is not noted until DDAL08-06 "Purging the Blood".
Artor Morlin returns in the subsequent Vampire Hunt trilogy (DDAL08-07, 08-08, and 08-09). DDAL08-07 "Into the Dark" calls him a "masked lord" multiple times, and mentions that his "endeavor is supported by Mirt the Moneylender, another masked lord". The NPC appendix lists his bond as: "He calls on his fellow masked lords for support and favors." The second and third modules in the trilogy each call him a "Masked Lord" twice.
Morlin is also featured in the later Undying Threat trilogy (DDAL08-13, 08-14, and 08-15). The first line of the background of DDAL08-13 "The Vampire of Skullport" begins: "The vampire MASKED LORD OF WATERDEEP, ARTOR MORLIN, [...]" The first episode also describes his mission: "Artor is a masked Lord of Waterdeep, as well as a vampire, charged with keeping any vampiric threat from the city." The NPC appendix repeats his bond from DDAL08-07, and a "Playing the Pillars" sidebar notes: "As a Masked Lord of Waterdeep, he is likely to have more resources at his disposal to deal with things like this." Morlin is merely mentioned in the second module of the trilogy. The third module, DDAL08-15 "Forge of Fangs", once again explicitly mentions that he is "one of the Masked Lords of Waterdeep".
Finally, Artor Morlin appears yet again in the Home Sweet Home trilogy (DDAL08-16, 08-17, and 08-18). The NPC appendix of DDAL08-16 "A Change of Address" once again explicitly notes that he's "one of the Masked Lords of Waterdeep", and his bond states: "I will keep my hunting to criminals and other undesirables so long as the other Masked Lords leave me alone." DDAL08-17 "The Tower of Ahghairon" notes the same, as does DDAL08-18 "Moving Day".
Though Dungeon of the Mad Mage is vague about the character's status, several Season 8 AL modules are much more explicit about it.
The wiki article also includes two other Masked Lords named by Ed Greenwood in this tweet from December 19, 2018:
Is there a current masked lord that would team up with Xanathar ? I wouldn’t mind an accurate name to say during my 6:30 game.
I’d not put it past either Berithro Tharlivar (jovial warehouse owner, shipper, and many-concerns investor) or Santar Belrune (relocated from Tharsult importer and VERY rich investor, fast becoming a Deep landlord of note).
These two don't seem to be mentioned in other materials.
The Masked Lords who died over the course of Death Masks (not all of them were assassinated) were:
- Avner Ravelmark
- Gorlar [forum post lists his first name as "Nammandus"]
- Barkheld Haelinghorse, human male [forum post spells his first name "Barkeld"]
- Ondreth Tolvur, human male smuggler and landlord
- Ammasker Gwelt, human male moneylender, landlord, and investor
- Braethon Cazonder, human male landlord and investor [forum post spells it "Braethan Cazondur", as does another FR wiki page]
- Landarmyn Voskur, human male shipping fleet owner and investor
- Belgantur Haelhand, human male smithy owner and retired swordsmith
- Oszbur Malankar, human male wine-seller and collector
- Dathanscza Meiril, half-elf female sorceress and artisan
- Ieirmeera Stravandar, human female
- Khaliira Arhond, human female
12 Masked Lords die in all.
Another section of the same FR wiki article lists the Lords of Waterdeep as of 1372 DR. One of them is a name that may be familiar to players new and old: Durnan, proprietor of the Yawning Portal Inn.
He's been portrayed many different ways over multiple editions and products (as detailed in this article by NewbieDM). He's still alive apparently thanks to potions of longevity found from his journey into Undermountain in 1302 DR (though Ed Greenwood hints there may be other reasons for it as well).
But most notably, Durnan was a Masked Lord of Waterdeep. This is noted in the AD&D 2e boxed set The Ruins of Undermountain and the D&D 3.5e sourcebook City of Splendors: Waterdeep. The companion who accompanied him during his journey into Undermountain,
Mirt (who's still alive in 1491 DR for a different reason - he spent almost a century trapped in a magical handaxe, per Death Masks),
is still described as a Masked Lord in material set in 1491 DR. However, there is no such mention of Durnan's own role as a Masked Lord of Waterdeep in any current material.
Waterdeep: Dragon Heist (p. 202) merely describes Durnan as "the owner and proprietor of the Yawning Portal", and says he "doesn't like talking about his past". Chapter 9 of the adventure, Volo's Waterdeep Enchiridion (p. 177), references his and his companion's adventure into Undermountain and then simply says: "Both used magic to extend their lives, but they eventually parted ways."
Waterdeep: Dungeon of the Mad Mage gives even less information about Durnan. It barely references his adventure into Undermountain in the "Dungeon History" section of the introductory chapter (p. 7). The closest thing that might be a potential hint to his continued role in protecting the city is the "reward" for one of the higher-level adventure hooks (p. 10):
An ulitharid (see appendix A) has been using telepathy to contact random people in the city and learn all it can about Waterdeep. [...] Durnan urges adventurers to find the source of the telepathic contact, learn its intentions, and put a quick and decisive end to it if those intentions are hostile. [...] If the party deals with the threat, Durnan promises to make sure the deed is never forgotten but offers no tangible reward.
Even so, connecting this to being a Masked Lord of Waterdeep seems like a stretch at best.
Finally, there's Tales from the Yawning Portal. As a collection of adventure modules from previous editions adapted to 5e, its canonicity to begin with is iffy at best - and of the included adventures, only Dead in Thay takes place in the Forgotten Realms by default anyway (some were originally set in Greyhawk, and others have no default setting). Regardless, Durnan is only mentioned in the introduction; the section on the Yawning Portal (p. 6-7) briefly summarizes his Undermountain adventure and gives a quick bio. Effectively, it just tells us he's "something of an enigma", which could mean anything (though the text characterizes him as more heartless/cynical than the Waterdeep adventures do).
Overall, the current material seems noticeably silent on the topic of whether Durnan is still a Masked Lord of Waterdeep. Whether this is an attempt to retcon his past, or simply left ambiguous, is for the reader to determine.