Facing is an optional rule on page 252 of the 5e Dungeon Master's Guide, and it does grant advantage to people attacking from behind. So, it's not just something Larian made up.
Technically true, but there are a lot of other components to that rule that Larian
didn't implement. Namely, the "a creature can change its facing as a reaction to another creature's move."
This component of the rule basically negates facing unless you are surrounded by enemies because you only have 1 reaction.