In D&D 5e do Dark Sun halflings still exist? Is it still part of the lore or was it scrapped with 4e?
There is not much official Dark Sun content in 5E at this time, and certainly no sourcebooks centered on it. So far we have official treatments for Ravnica (Guildmaster's Guide to Ravnica) and Eberron (Eberron: Rising from the Last War). The Forgotten Realms got one sourcebook focused on a part of that world (Sword Coast Adventurer's Guide).
Mordenkainen’s Tome of Foes is named after a Greyhawk personality, and has a planar theme to it, but it's pretty generic. There is a sidebar on Halflings in Athas, but that's about it for Dark Sun content:
On the world of Athas in the Dark Sun setting, halflings are feral creatures, prone to devouring the flesh of humans and elves. Small, furtive and sun-bronzed, they live a hard life under their chief, eking out a savage existence by hunting, foraging, and raiding. Outside their tribe, halflings are mistrusting, cynical, and often paranoid, since they think that all other races are as fond of devouring humanoids as they are.
All the adventure books to date have been set in the Realms, except:
- Tales from The Yawning Portal - a rewrite of old content, though the Yawning Portal itself is a Realms location.
- Curse of Strahd - set in Ravenloft.
- Ghosts of Saltmarsh - natively set in Greyhawk. It has notes for adapting it for use in other settings, but Dark Sun is not mentioned.
None of them are easily adaptable to the blasted desert of Athas. Princes of the Apocalypse mentions Athas as a possible alternative location, but doesn't actually provide any guidance on how one would go about doing so.
There is nothing in the lore of Athas to suggest that Halflings have any reason to go anywhere. No official conversions have been provided for 5th edition, however the previous upheavals of Athas ecology have not been naturally occurring events. Given 4e seemingly chose to pick up somewhere in the middle of the Dark Sun novels (as best I could tell from the details provided) and 3 RPG books (setting, bestiary, adventure) and 3 non-sequential novels were ever published for 4th edition trying to figure out where 5th edition might pick up is a losing proposition.
That said, as of 4th edition, the halflings were still very much a viable race and if you choose to remove them you should give serious consideration as to what mechanical or thematic benefits that actually provides as well as what the cause might be.
To the best of my knowledge (ie: checking the WotC site and doing some searches) I haven't heard of an official Dark Sun conversion being done by WotC.
shrugs It is possible I missed it.
Athas.org is the official site for 3.x Dark Sun and is considered first party material due to the co-ownership clause regarding content in the site's charter. They have a unofficial 5e conversion taking place over there.