Flanking rather than backstabbing. Flanking advantage is a result of an opponent being attacked from multiple directions making defending difficult. Backstabbing is a result of the turn based combat and does not make sense in a cinema of the mind type of context and thus breaks immersion. Also flanking is in the rules while backstabb(mid combat is not)
Flanking is an optional rule in the DMG. It makes totem barbarians and rogues overpowered because they can claim advantage just by setting up a flank. You also get a really silly conga line fight that looks nothing like a real fight. I tried this rule whilst DMing tabletop and it ruined the game.
Flanking works in Pathfinder and 3.5 D&D because there are other rules that interact with it and balance it out. When you just slap it on top of D&D5 you ignore the fact that lots of classes have abilities that get a green light only when they get advantage.
