I cited my source, 1E Dungeon Master's Guide. Here is some more information: Page 39, left side, under the title "Acquisition of Magic User Spells". That's enough for me, 6th level is a high level according to the great Gygax, who has played more hours of D&D than probably all of us put together. Does it make you wonder why would he make a statement like that? Could there be an underlying wisdom that Gygax gained from all those hours playing the game?
When I played ToB, I found the transition between level 20 and 21 to be much less consequential than, for example, between levels 5 and 6 (which we now know is a high level) in BG I. And the excitement between levels 27 and 28 was even less notable. I'd rather just start a new game, were it not for the story needing to conclude. I think Frodo Baggins ended up around 6th level or so when he returned to the Shire.