Reduce what the DM needs to know
It seems your DM is regularly asking about Player Character features. Why is that their responsibility? Sure it seems like something they should know, and outside of the session I would wager they do know it. However you may be underestimating the number of different things a DM is trying to keep track of during a session.
During a typical encounter a DM of a group of four needs a functional understanding of:
- Describing the setting and encounter
- Tracking initiative
- The actions of between 1 and 5 unique NPC or monsters (likely with once off abilities to learn on the fly)
- The locations, reach and movement range of all creatures in combat
- The hitpoints of all creatures in the combat (and optionally PC's)
- Managing music, lighting or other atmospheric tools
- Maintaining a narrative and pacing of the story
- At least one level 15 caster's shenanigans and how they impact the NPCs
- A level 15 martial class with some kind of aura or defensive ability that impacts creatures near them
- The level 15 Gloom Stalker Ranger / Rogue multiclass with a complicated sneak attack calculation that may or may not include sharpshooter
Can you see how in the moment the DM might not remember the exact mechanics for your PCs? Most players struggle to remember everything their 1 level 15 character can do, why expect a DM to remember everything for a whole party of them?
Taking Responsibility for your Characters
Instead of viewing this as "How to help our DM remember?" view it as "How to reduce the number of things our DM needs to remember?".
To start with, each of you should take responsibility for knowing your own characters abilities. Be ready to explain to the DM any rolls or circumstantial effects they need to be aware of. Revisiting your example:
- GS: I'm going to attack the Gnoll. Sneak Attack!
- DM: How does that work?
- Players: Why don't you know?
- GS: Do I have advantage? If I do I get 4d6 extra damage.
- DM: No, why would you have advantage?
- GS: I have Umbral Sight, so I'm invisible meaning I should have advantage, oh and sharpshooter!
- DM: Ok, so you do have advantage, what does sharpshooter do?
- GS: rolls 15 - 5 from sharpshooter, plus 7 to hit. Does a 17 hit?
- DM: That hits, how much damage?
- GS: 27 + 10 from sharpshooter.
Consider how that could be improved if the player didn't expect the DM to know everything.
- GS: I'm attacking the Gnoll with sharpshooter. I have advantage from Umbral Sight so I get sneak attack. 17 to hit?
- DM: Hits, roll damage.
- GS: 37.
It sounds a bit formal written out like that, and you wouldn't really talk like that at the table. But the point is that by giving the DM the relevant information upfront they don't need to worry about remembering it.
This requires a solid trust between players and DM that no one is cheating, but if that is a concern at your table this isn't the question you should be asking.
Offer to help
Another way to help your DM is to take some of the load off them. If they are open to it, you could delegate things like tracking initiative to one of the players. Similar managing things like music and lighting (if used), moving miniatures or calculating range are all things that can be handled by a player. What your DM wants or needs help with is something you should discuss with them.
Whatever you decide, try to cut your DM some slack. They are doing a hard job and remembering the specifics of your characters is the least of their responsibilities.