To be fair, a large part of this is that AD&D saw a lot of different designers over the years (particularly in the 2E era) who disagreed with earlier strictures and worked to break them down, introducing exceptions where previously there had been none. Which isn't to say that they were deliberately trying to break from convention, or had any sort of bone to pick with established prohibitions (or even understood why they were there), but the end result was the same: more and more exceptions could be found if you knew where to look.
The
"leatherette" books were particularly notable for this, as they had a lot of exceptions to the demihuman class restrictions that up until that point had been (fairly) inviolable.
Well, unless you believe Unearthed Arcana is real, since as I mentioned, it had some wackiness:
Half-Orc: C/F, C/T, C/A, F/T, F/A
Halfling: C/F, C/T, D/F, D/T, F/T
Dwarf, Gray: C/F, C/T, C/A, F/T, F/A
Dwarf, Hill or Mountain: C/F, F/T
Gnome, Deep: C/F, C/I, C/T, C/A, F/I, F/T, F/A, I/T, I/A
Gnome, Surface: C/F, C/I, C/T, F/I, F/T, I/T
Elf, Wild: F/T
Elf, Dark: C/F, C/R, C/M, C/T, C/A, F/M , F/T, F/A, R/M, M/T, M/A, C/F/M, C/F/T, C/M/T, F/M/T
Elf, Others: C/F, C/R, C/M, C/T, C/A, D/F, D/R, D/M, D/T, F/M, F/T, F/A, R/M, M/T, M/A, C/F/M, C/F/T, C/M/T, F/M/T
Allowing Multiclass Bards was about the weirdest thing in the leatherettes I remember, and something I always wanted to do but never got around to. Bard/Mage? Sharing Wizard spells in your spellbook
with yourself? Sounds wild.
But some limitations were apparently so inviolate, so core to the D&D experience that you got things like the Compete Book of Dwarves which had to make a Fighter/Cleric Kit just to say "Of course Dwarves can't be Paladins! But uh, here's a bunch of Paladin abilities for your LG Fighter/Cleric..."