Future Sight should not be banned as a move. It's not broken on it's own. It is actually a healthy good move that allows teams to force progress against fat. What is being discussed is Future Sight combined with Teleport. I'm not even sure if it's broken on Slowbro but leaning towards no. Urshifu is already unwallable if it runs the right moves. Future Sight does make prediction easier if your hitters are choice locked I guess. I've never felt like I needed Future Sight Slowbro on my teams. In fact I only use Future Sight when I use Slowbro because it's a move that Slowbro gets. I don't use Slowbro because of Future Sight but because it blanket checks a lot of Pokemon and has Regenerator to stay alive trough out the game. The argument that it makes Pokemon unwallable falls flat to me. A lot of Pokemon can become unwallable if it runs the right moveset/Item. Here's just some of them.That's true. If anything, I'd suggest just ban future sight. That's the only thing that makes Slowbro an incredible pain in the ass. That isn't a too complex ban at all since the move single handedly puts a lot of pressure when used. Complex bans would probably just cause some random arguments like you said like why ban that and not this but just banning one move is a lot easier. They did it with baton pass since it is incredibly overpowered when speed boost is passed into a slow wall breaker. Without future sight, Slowbro would just be the same old where it walls stuff like ground types only this time, it can be a good scout with teleport
Urshifu with Poison Jab and Aerial Ace
Victini with Glaciate, Scorching Sands + V-Create and Bolt Strike.
Pheromosa QD with Shock Wave or Drill Run on Physical sets.
Tapu Lele with LO. Lele is so strong, people usually have to play around it and hope it's choiced in order to force it out. LO fixes this and if you want the full power of Specs just run Modest.
Nidoking with Superpower
Garchomp with Dragon/Fire/Ground coverage.
Dracozolt with Electric/Meteor/Fire Blast
So yeah you definitely don't need Future Sight and it is NOT the reason people use Slowbro. It's more of a bonus thing.