Per The Fediverse Report:
Tumblr has reconfirmed that it is working on connecting to the fediverse. In late 2022, Automatic CEO Matt Mullenweg said that the site was going to add ActivityPub support ‘soon’. Plans changed for Tumblr, including staff layoffs, and for a long time it was unclear if this plan was actually going to happen. In summer 2024, Tumblr announced that they would be working on moving the backend of Tumblr to WordPress. In an AMA this week, the company said that this migration of Tumblr to WordPress means that Tumblr can also use the plugins of WordPress, including the ActivityPub plugin. This means that people will be able to add ActivityPub to their Tumblr blogs. Not much is known about how this would work in practice.
Let’s see. Not holding my breath.