I'm not sure why, but I feel like I've seen this topic come up quite a few times recently on various boards and - near as I can tell, anyway - there generally seems to be an attitude of 'well, that's not actually an issue since (theoretically, anyway) anyone can make their own 'sub" on another instance (and potentially even defederate from the instance hosting the undesirable version altogether, maybe?)
Personally, I question how effective such measures would/could be when scaled to thousands - maybe millions of regular users, not to mention intentional malicious entities like bots, etc...