I think it has to do with Apple - and Apple pulling the tumblr app off the App Store due to child porn.
All my posts have now been individually flagged - they're still there - but as of 12/17/18 my understanding is they'll all be marked "private" - and no one will be able to see them. I don't think they are actually deleting posts. Though they could delete tumblrs.
I had an account on Blogger - and I just reactivated it. Blogger is owned by Google - and for about 2 weeks in 2015 or 2016 they said they were going to ban NSFW posts - but they reversed the decision before it went into affect. So for now I plan on going there. The list of NSFW blogs I use to follow is still there - and some are still active.
My advice would be to check out Blogger, Wordpost, Reddit (there's suppose to be a NSFW section there).
My guess is that tumblr will lose so many users that they'll shut. But WTF do I know .. ?
BUT PLEASE POST WHERE YOU ARE GOING SO EVERYONE DOESN"T LOSE TRACK OF YOUR TUMBLRS!