Plans to add ActivityPub support for Tumblr are likely dead. I can’t see that as a priority given Tumblr’s recent drastic staff cutbacks.
Rodrigo Ghedin: Automattic’s Tumblr/ActivityPub integration reportedly shelved.
Laurens Hof: ActivityPub support was a “hasty announcement” by CEO Matt Mullenweg, followed by “quick quiet shelving of the project only a few days later.” Tumblr is supported by ads and subscriptions, and interoperability undermines both those business models.