Thundaar's quest still sometimes assigns very rare monsters.
He just assigned dark elven priestess, which has 25 rarity. Savegame attached. I've received this quest several times in p21, and it always gave me 50 rarity monsters before that.
elf.svg - original pre21 save right after quest
elf.bak - orinigal backup several levels before that
elf.svg (2) - restored from backup and moved near Thundaar, without taking quest
I've tried several more times - Shadow Lord (25), Revenant (10), Chaos Servant (10), Green Blob (25), Giant Bat (100) were assigned. Something is seriously wrong here, in pre20 dwarf almost always gave out (25) rarity monsters, and in pre21 it supposed to give out only (50) rarity monsters.