That's basically the "flanking" we talked about so often.
I would be all for it as a replacement for the stupid "free backstab" we used to have or the silly "hide as a free action without a check" we are left with now, but as far as I remember it was considered an optional rule, so many consider it divisive.
The "you don't need advantage if another enemy of the target is within 5 feet" rule is implemented in the game and has been from day 1. Though most people never noticed because until now backstab was a thing. After the new patch I've read some reddit posts reporting a bug that it doesn't always behave as it should, personally I haven't run into any problems yet. I hope they fix it, it's one of those things that are super important for rogues and also make the game feel like legit D&D.
"Flanking" is a different thing. It's an optional D&D rule (that a minority of players use at their table) where you get an advantage on your melee attack roll if you've got an ally who's also in melee with the same enemy but positioned on the opposite side of them. It's a callback to a similar rule back in 3rd edition D&D that many people enjoyed at the time, but in the context of 5E it's a little overpowered. BG3 doesn't currently use flanking rules.
