For any Tumblr users here, this has already rolled out completely unannounced and is opt-in by default. You need to manually opt out, which can only be done on the desktop website. Odds are good that your data is already being sold to Midjourney and used to train their models.
To do so, click on your blog on the sidebar, click on Blog Settings on the other sidebar on the right, scroll down to the Visibility section, and turn the "Prevent third-party sharing for [your blog]" toggle to ON, not off. If you have any sideblogs, you'll need to manually do this for each of them as well. It's per blog and not account-wide.