If the game is claimed to be based on a particular edition of the rules, and the developer creates a rule that differs from that, then it is their homebrew, regardless of if that rule has existed in other earlier editions. In this case, BG3 is (loosely) based on 5e, which does not have ciritical failures or critical successes, except for attack rolls and death saving throws. Natural 1s and natural 20s have no extra effect anywhere except in those two specific cases. Critical failures and critical successes on ability checks (5e does not even have 'skill checks', formally speaking) and saving throws is, in this case, Larian homebrew because they do not work that way in the ruelset upon which the game is based.
