You can challenge a person to NatDex Anything Goes (as Dorron noted), so it still exists in that sense, even if it lacks a ladder.
The ladder isn't coming back up anytime soon. It's an eyesore in battling notorious for super unfun, cheesy, annoying gimmicks like ReviveCats, and its an eyesore in coding because it's built on hypothetical guessing how mechanics never sharing a game (e.g. Mega and Tera) would interact together. (To be clear, this means it is not a fault of the coders, but just that nobody knows how certain interactions are "supposed" to work.)
I still disagree with its removal despite all that, so like I get you, but I'm in no position to do anything about it anyway.
Any limits on teambuilding for an AG format are never going to happen. It's just not Anything Goes anymore at that point. Any restriction that you think makes the tier fun again, 99% will not work the way you think it will (eg your idea leaving ReviveCats untouched). Competitive optimization is inevitable when you let players build teams.