I know this is probably very niche but does anyone play beyond level 20?  The current rules cater for it I think by allowing you to get additional epic boons rather than a character level having achieved a total of 20 character levels.  The older versions of the game allowed you to get a few levels of this and that and continue indefinitely with your final class (eg. you could be a 3rd level fighter before becoming a 21st level wizard).  It just gives me the irrits knowing that if I take a couple of levels of something I can never reach the potential of ones ultimate class (theoretical as it mostly is 😉).

After 20 it's not so much about leveling up even more, but perhaps working on finishing the campaign and killing a god or three.

I've discussed with my DM that I am interested in playing that far ahead because you still want to do a few sessions with some new toys that you get. I don't know what everyone gets at level 20 (Some quick googling tells me a rogue can convert a miss into a hit or turn any roll into a 20, both things are nuts) so it's really not about continuing into levels but killing some tough enemies, getting some powerful artifacts, and using those to kill another powerful enemy until the campaign itself is properly done.

According to almost every bit of market research WotC have done most people don't play beyond about 12th level, never mind beyond 20th. I'm sure there's a few groups out there doing it but as you say it's very niche 

I have played at a few 2014 5E tables that went to 30.  Their rule was start leveling up in another class that was related or that you had story reasons to pick. A wizard who became very pious during their adventure could start taking Cleric levels or Artificer levels. Cleric because of story behavior, and Artificer because of casting stat.
A few picked up a second subclass but could only advance to 10 in it, but you had to clear it with the DM.

My current table is still debating how to go about higher than 20 in the 2024 rules.