As a note, in the original AD%D rules, the level limit of 20 levels did not exist. Instead, level max was based on race/class combo. Human, regardless of class, had no limits while the demi-humans (dwarves, elves, halflings, half-orcs, half-elves) all had limits based on what classes they were. The demi-human class limits were mitigated by the fact that they could multi-class, thus allowing them to act as 2-3 different classes at once. Humans couldn't multi-class, but did have the ability to dual-class. Basically, they would originally level as one class (such as warrior), then switch to another class. The abilities of the original class were unusable until the new class was at the same level as the original.
I do remember there being a high-end adventure module that was actually designed for level 100 characters. Yes, level 100. Had some seriously nasty stuff going on in it. Had to know what you were doing to get through it.