That was part of flanking, but It's more complicated than just adding facing because adding them without it amounting to permanent advantage if you have more than one melee type in the group requires rebuilding all the missing AoO rules. Take this example fightFacing is trivially easy to replicate, after a fashion, simply by ruling that a shield can only be effective against the first two creatures* that attack you in a round. This way you don't have to fuss with actual directions and facing etc. if you don't want to or if you're doing TotM, but the general idea of "a shield only goes so far" is maintained.
* - swarms being a different ball o' wax altogether.
In 5e if you add flanking, A&B can just swing around to the left & right of C while completely ignoring D without suffering from even a single AoO resulting in both flanking and facing to be games with no risk or effort despite the huge power of ignoring a shield or gaining advantage because 5e allows you to use your full movement (or even dash for double)to run circles around a creature without suffering an AoO unless you move away.
So because they did away with any meaningful tactical impact of AoO's on the battlefield you could say anything but a 5 foot step when moving between threatened squares, but now you need to consider if cunning action & every other ability that lets you move in combat needs a special exemption or not & while your at it should preemptively go through every action & ability to decide what now provokes an AoO before your players start asking till one day they ask for a list. Later on you get a new player & hand them a spiral bound booklet of houserules saying chapter one covers AoOs & changes to class abilities for them.
