Comment by apitman

You're right, in default configurations ActivityPub definitely has an advantage here, since HTTPsigs are tied to users, not instances, which gives finer grained blocking. I'm not aware of anything like this for Webmention.

I suspect this is because WM is used far less than AP. It also grew out of a community (IndieWeb) where having your own domain is a core tenet.

I think something like Mastodon could work with WM though, since all URLs hang off of user URLs, so you could block by URL prefix, ie "block https://example.com/user1/*".