5e rules don't actually have flanking at all, but some form of house rules for flanking is pretty common. The most common I've seen is that flanking attackers get advantage, which sometimes feels a little too strong. I play in some games where flanking attackers get +1 or +2 on their attack rolls, which feels a lot more reasonable. Advantage is a pretty big deal to give away that easily (not that it's always easy).
Same in my games (as a player). It is a popular homebrew (+2 for Flanking) especially after some D&D Youtubers made it more popular (which is where my DMs saw it).
Reason : both players and DMs felt flanking was too easy to get Advantage on NPCs that easily end-up outnumbered. In 3.5, running around someone could provoke an AoO, so there was a risk in some cases.
They could also give a +2 to attack rolls from higher ground, instead of advantage. Would be so much more fair for melee characters and make it easier on the squishy mages and such.