Mastodon has a concept called "pinned statuses", which is a special collection attached to a Person actor.
https://docs.joinmastodon.org/spec/activitypub/#featured
It wasn't readily known how this collection is updated and federated (not without code achaeology), but claire@social.sitedethib.com recently shared some additional info :smiley:
The actor itself will issue an Add activity targeting the collection with the status in object.
This activity is sent to all followers of the actor.
No activity is sent if the actor has no remote followers.
A Remove is sent when a pinned post is unpinned.
This is what the Add looks like:
{
"@context": "https://www.w3.org/ns/activitystreams",
"type": "Add",
"actor": "https://example.org/users/testUser",
"target": "https://example.org/users/testUser/collections/featured",
"object": "https://example.org/users/testUser/statuses/115266412340579560"
}
The corresponding Remove is identical except for type, which is of course, Remove.