Update on the tumblr situation two weeks after the supposed purge: The change of policy has successfully driven away many of the creative people that made tumblr special, but so far (I last checked on Dec 31st) it has spectacularly failed to cover up most of the NSFW material.
My impression is that the AI is clearly failing to keep up with the sheer bulk of stuff swilling around on tumblr - I like to think that is just so busy with the photos that it hasn't got round to watching gifs and videos. Moreover, its sex education appears to be stuck in pre-school. It can recognise a female-presenting nipple in good light, but it is entirely clueless what people might be up to with their hands down their trousers, for instance.
Thus, watching tumblr's AI learning about the birds and the bees would be immensely fascinating if it wasn't at the same time in the business of killing an entire culture. I've spent way too much time throughout December testing the limits of the new system and making sure the interesting materials that mysteriously remained unflagged find wider distribution. But as of the new year, I'll take a bit of a break from that and look into alternatives, as tumblr is going down anyway, no matter how hopeless its censorship AI is.
So far there is a silver lining in that the rescue site 2mblr.com seems to be working as an uncensored archive, although it has no connectivity yet. Some have suggested it's a scam but so far it works for me, so let's hope for the best. Just under half of my main tumblr has copied across and is now to be seen uncensored (including lots of protest nipples from December) here. (I now also copied Street Music Oxford) Recently, an archive mode has also materialised, and just now my page features 4 "zombie posts" highlighted at the top, so things are definitely happening at 2mblr. (Update 7.1.: the site now has a follow button, network building starts here!)
Even my poor old painter smurf got flagged by the tumblr AI. This programme has no sense of humour and no appreciation for art.