The worst part is, the data has been scraped already, regardless of any opt-out, and there is no explicit confirmation that it hasn't been shared with midjourney already. The wording on the staff post is... Very Vague about that.

Because if that's the case, no amount of opting-out will change anything. Tumblr says they'll notify Midjourney if some data is now opted-out, but come on, I have absolutely no reason to believe that Midjourney will do anything about it. They don't care, they already have the data.