We have implemented group federation, and ensured compatibility with Lemmy (MR #2100).
This was achieved by strategically introducing the Blog Plugin.
Related discussion concerning compatibility with Friendica: Lemmy issue #2144.
Both GNU social and Lemmy have created test fixtures and the respective tests to ensure that this now existing compatibility isn't loss. More fixtures are in the plans.
Various improvements and changes in our Notification component derived from this development. GNU social and Lemmy have also agreed on expressing and understanding unlisted Likes in a manner analogous to how we handle Notes. But GNU social still has to implement support for that form of Like.