@Gargron should https://websub.rocks be working with an atom feed like https://mastodon.matrix.org/users/matthew.atom? It seems to subscribe okay, with an entry in the subscriptions table, but never gets any pushes. Similarly when testing with npm's pubsubhubbub lib. Is a subscription secret mandatory?
Hello world :) Just stood up a Mastodon instance for Matrix.org with a view to bridging between Mastodon & Matrix!