My concern here is spam. With hindsight both Pingback and Trackbacks (which Pingback was a response to) essentially became APIs fit spammers to add junk to your site. Can webmention avoid the sane fate?
The README says that one of the steps is verification [https://github.com/converspace/webmention/blob/master/README...], but it's a little disappointing that it's essentially an "exercise left for the reader".
We did that in pingback too. It turned out to be trivial for spammers to circumvent.
If I were to implement pingback or webmention today I'd use a moderation queue with the ability to allow-list trusted domains so they get to skip moderation in the future.
The typical verification step (check if the originating site contains a valid link) is trivial to work around. One attempt to address that is Vouch [1], but I believe it's largely untested in the wild.