Smogon has voting sessions, and a majority vote can ban a pokemon from a tier. I'm curious who the voters are? I used to read the chat of a league I gen for, one of them kept talking about getting the "reqs" to vote on pokemon. I wonder if those people have personal agendas. This generation, Baton Pass was banned completely to Ubers or even AG, and I remember reading Smogon's statement being that it's not strategical or something? Also, Arena Trap was banned to Ubers, making Kanto Dugtrio with that ability Ubers. That mon, but as you say, Tapu Lele isn't banned from OU? Remembering those odd bannings, and reading this, I can see that there is some real bias in getting certain mons off OU and keeping some in. Probably what happened with Greninja in Gen 6. People hated dealing with it in OU and exaggerated its threat to get it to Ubers. Now it's OU as it belongs. I know some leagues follow their own tears, and at least from Gen 6, there was a "GBA" tier system, based on the league with the same name. I do like having your own tiers, a small group of people having a league and doing that. Smogon sometimes is bullshit. That being said, I love that competitive play largely banned accuracy/evasion modifiers, because in battle facilities, fucking Double Team is annoying.
As much as I hate battling Darkrai before Gen 7, I don't like that Dark Void got nerfed.