We didn't hold to the level limts. Oh we tried a few times but when we reached them and were in position to exceed them it just seemed too arbitrarily cruel not to allow the player to continue.
What we DID observe quite faithfully (a very few singular exceptions) were the allowed classes and multiclass combinations. It's one thing to be told, "Don't even start." It's quite another to be told, "Now that you've come this far and are still having fun - STOP."
In my experience, it was the opposite way around:
We'd hold to the level limits, but allowing every exception case we could come up with, like using the optional rule for higher level limits for high Prime Requisite scores, and we generally allowed Wish and Limited Wish effects to allow characters to gain extra levels (1 level above the normal limit with a Limited Wish, 5 above with a Wish).
The allowed classes & multiclass combos were right out the window. Pretty much any race could be any class, as long as they had the ability scores and alignment. Just about any multiclass combo was allowed as long as they didn't have more than 1 warrior class (Fighter/Ranger/Paladin), 1 Priest class (Cleric/Druid/Monk/Specialty Priest), 1 rogue class (Bard/Thief/Assassin), and Psionicist was it's own category. You couldn't have more than 3 classes, and if you wanted to stop progressing in a class permanently (we called it "barring") you could choose to do so and not put XP into that class (typically because you were at the limit for that class and any further XP would be a waste). . .but it would take a Wish to be able to proceed any further in the future.
We created level limits for new class/race combinations, and since Humans could be unlimited level in any 2e class, and up to 17 in Monk and Assassin (IIRC), and Half-Elves could be unlimited in Bard, we added classes that had unlimited progression to all the core races. Elves could be unlimited in Druid. Dwarves could be unlimited in Paladin. Gnomes could be unlimited in Necromancer. As a general rule, we assumed that any race could rise to at least 9th level in any class unless it was a strange case.