This. It lasted all of 1 session, and that was with rules that made it harder to get (leaving a square provoked, and the 5' step was a bonus action)We tried Flanking at a couple different level points and got rid of it. It made advantage too easy to get, and really trivialized features that granted Advantage.
Also this.(And this was a while ago - before Elven Accuracy which would be even more powerful, and the Samurai fighter subclass that would be mostly useless.)