I don't know how it'd work but I'd be interested in something to deal with spam/scams. That annoying "Fediverse chick" thing, sure i blocked her, as can other individuals. And I guess the account could be flagged to whatever instance the account is registered to? But if it became a frequent problem, with bot account spamming people, it would be handy to have a way a tracking what accounts are getting blocked by lots of people.
Even if I wouldn't want to autoblock accounts just because they're unpopular, I might want to stop or mark as 'caution' private messages from "problem" accounts.
I'm sure FOSS apps are less intrusive, but they still use notifications surely? I'd generally want a calendar or messager app to send me notifications, but I might want to block a specific app depending on my use case.