Hero Pools (as they were officially called) were stopped in the public matchmaking games for a variety of factors. Most notably lack of player engagement and the alledged diversity of hero balance changes made through the Experimental Card (feel free to laugh at that second part, I don’t blame you).
In the 2021 Season of Overwatch League, the second and fourth tournament stages applied a fixed hero pool throughout the entire stage banning 1 tank, 1 support, and 2 damage roles. This hero pool rule did apply to some regional Overwatch Contenders tournaments, but did not apply to the live game.