No, but that's not to say that they can't.
5th edition doesn't technically have any level progression after 20th. All of the things that the DMG recommends - boons, ability score increases, additional feats - happen once you gain 30,000 additional experience.
That being said, if you are a DM and you want to create a structure for epic levels (or levels beyond 20th), I would recommend increasing the character's proficiency bonus by 1 at 21st level, and by 1 every four levels after that. This fits with the PHB progression - your proficiency bonus starts at +2 and increases to +3 at 5th, +4 at 9th, +5 at 13th, and +6 at 17th.