Comment by apitman
I might be misunderstanding what you're saying here. How is ActivityPub more authenticated than Webmention? WM requires the poster to host their content on a website. This is exactly what the AP spec says to do. Now, since the spec was published, most AP implementations also support HTTP signatures[0], but this doesn't provide additional guarantees that you can't get with WM. The authentication is still tied to a URL.
As far as spamming goes, I don't see how WM is any worse than AP. In both protocols your only options are passlists and/or blocklists.
[0]: And an old version that doesn't have an official spec. ActivityPub's issues with spec stagnation and de facto standards is a whole other thing.
I haven't dug into these, so apologies for the naive question, but for a multi-tenant service like WordPress.com, can you effectively limit which WordPress blogs can WebMention you? If the allowlist is formed on the domain, this seems limiting.
Perhaps more advanced URL regex can achieve more fine-grained control but I do still see advantages in pubkey auth (especially if people want to move their content.)
Still, I do find myself wishing for a lighterweight-than-ActivityPub middleground.