my apologies for starting jocular.
E6's top level doesn't advance, as in there is No Level.
It doesn't have to be level 6, it could be any level, say randomly determined d12 let's play that then No Level
s.
wotc put out a series of class articles about Dead Levels, where there's
a level but it's the least of one, so they add a ribbon to it to make it feel more meaningful than 1 level of commoner.
5e distills most of the various level bonuses out of view, and ignores the previous dead levels position for full casters, effectively saying a worse-than-dead-level is now "A Level".
C.O. has either proved or near proved, that top end levels the math breaks down. People play up there sometimes, but stuff just goes wherever/whichway off track. They are levels, and they're Meaningless Levels.
If a particular game has enough magic tea party going on, the whole idea of levels becomes of little importance. It's kinda like all the pages in the dmg that are worthless in a low magic item campaign = thanks for taking my money I like being ripped off with a smile for the "art". These meaningless levels are somehow quite different from the previous meaningless levels just above.
They could be proxies for people real life Age Levels. Dude says I'm level 30-something, I can scare level 11 goblins anyday besides halloween.
1e had Level 0 npcs. Squeezing down all the details of the red shirt lieutenant to a well deserved 0. Hey I got bit by a bug <player drops> {end scene}.
Military , 1e class level names , and Modrons , might as well use a Number Level, right? Where the number subs for whachamacallit Fluff you'll fill in later.