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/*".
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/*".