They just added GoToSocial support [0], and they are adding support for more instance types [1]
(I wish the FediActivityPub people had spec'd up an API to avoid this kind of multi-implementation-who-supports-what shambles.)
EDIT: Amusingly, I fixed the "cannot login to Pleroma" problem for `madon` this week - Pleroma/Akkoma require form data in the body, not the URL, for POST requests (which is fair since the HTML spec suggests this is the Right Way.)