Props to Chord for beating me to it.
For years now, blogging website and degenerate cesspool Tumblr.com, a website comprised of blogs mostly relating around fandom, art, social justice, weirdly niche drama, aesthetic, stupid memes, and other general enjoyable nonsense, has had an extensive problem with pornography.
That is to say, it's had an absolutely outrageous problem with link-spamming pornbots.
The consensus among the user base is that actual erotica blogs run by actual people are copacetic. A large chunk of the Tumblr crowd are artistic types, and so spicy fanart of beloved characters, artistic photography with nude models, depictions of sexuality for educational or sex-positive discourse purposes, regular naked art, and yes actual porn, were all deemed fine by the community, as long as those posts were appropriately tagged. With things "tagged", the Tumblr method of organizing content, other users could either specifically look up those images, or they could block the tag so that the content would never come across their dashboard (their landing page of content). If you wanted a Nice Christian Tumblr devoid of raunchiness, you could avoid the carefully curated and human-managed smut.
Those blogs aren't the problem.
The problem comes from the locust-like swarm of automatic bots that flood the comments of any post they can get to, regardless of content or audience, which are a fucking menace. Check the geology tag-- "Khristi loves Big Cocks! Click here~" Check the animation tag. "Click here for SLUTS!" Check the comments on a meme about Batman. Check the comments on a post about grandmas making knitting patterns with excel. Check vine compilation vids. Pornbots. Pornbots as far as the eye can see.
And it wasn't enough for the bots just to make comments on posts. The bots did their best to permeate every available aspect of Tumblr they could. They'd reblog posts, follow users, spam-message users privately-- any part of Tumblr that served as an even a semi-public forum was vulnerable.
People complained about it for years. At first, the @Staff's stance on the matter was to ignore them-- back before there was an ignore feature. Then they introduced ignore and block functions to block individual users/bots. . . Which did nothing to help the problem. It was akin to an individual squashing a single roach while being swarmed by more roaches, and for that aforementioned squashed roach moving on the another swarmed individual in the hopes for Hot Sex Now!!
Reporting, likewise, was a crapshoot. Tumblr simply didn't take the issue seriously, or if it did, it did nothing to show it.
Origin of the Bots
Side note. Some may wonder why the hell bots would flock to Tumblr to begin with. It goes something like this.
Essentially, Porn Site Joe wants his porn site to be the best in the google search algorithm, but it is clearly just a virus filled cesspool. So he sends out an army of bots to link randomly around the internet. If his site is associated with Good Wholesome Sites that are verified as legit by the Google Algorithm, his site gets an algorithm boost and shows up more in searches. However, most websites have better site security than Tumblr does, and so knocks those bots down like swatting flies. In comes Tumblr. With Tumblr's near non-existent defense against bots, the bots can come in, make oodles of fake accounts, and connect to otherwise legitimate "sites" in the for of Tumblr blogs via commenting, following, liking, messaging, and reblogging.
It doesn't even matter if people click on those links; as far as Google is concerned, it sees Shady Joe's site linked to good wholesome Nerd.Dot.Tumblr, and sees that Nerd.Dot.Tumblr's site is linked to other wholesome Real People blogs that reblog and comment and are generally existing as actual human entities. Therefore, Shady Joe's site looks a bit more respectable, and gets some points in the Google Algorithm game.
The Very Bad No No
Cut to a week or so back, and the Very Bad No No happens. Tumblr's app is pulled from the Apple Store because of child pr0n. According to the staff, they had their own bots and algorithms in place that match images to an FBI database of abused children, and such images would be automatically deleted. Apparently somebody dropped the ball, because that content made it onto the site after all, and was enough of problem that Apple took down the app from the AppStore.
Finally! the userbase cried. Will the @Staff finally do something? Will they put in better measures to ban the bots? Will they take security more seriously?
Nah. It's just easier to wholesale block anything remotely adult and a lot of stuff that isn't.
The Bots Strike Back
In an act of either amazing misunderstanding or outright malevolence towards their user base, Tumblr decided to ban anything and everything it deemed to be adult content. Supposedly, artistic works and educational materials are still fine, with just outright porn being blocked. According to the @Staff notice, they realize that there is a distinction between unseemly human gratification and actual art, and they will do their best to respect that difference.
Now, that at first doesn't sound so bad. Just avoid being an actual pornography blog, and you're fine, right?
Wrong. Despite its claim to leave artistic works alone and target only actual raunchy material, Tumblr's newest porn blocking bot has proved itself to be just as incompetent as the site management, and has been targeting content at random, including safe for work art, dinosaurs for Scholastic kids books, cubism, Mario, memes, those tasteful artistic renderings that were supposed to be safe, this bird, ancient Egyptian murals, and itself.
The leading theory is that is it using a "skin tone" filter, searching for things that are flesh colored and then running that through an algorithm to determine how much flesh is too much flesh. People have been playing with that information.
Furthermore, entire tags of content -- Tumblr's main method of curating user-generated content-- that are otherwise safe and wholesome are being wholesale deleted. Notably "chronic pain" and "lgbtq" were deleted, incensing the disabled communities of Tumblr and the LGBTQ communities who used those tags for discussion or comradeship.
Now, I would be remiss if I did not acknowledge some of the slightly more tin-foil hat theories going around regarding Yahoo and Tumblr. The theory goes that Yahoo has been planning to tone the site down and make it more "child friendly" for ages, and has already been shadow-banning content, and that this Apple fiasco was the incentive they needed to really kick down the doors and clean house. There is also talk about what kind of ironclad grip Apple has and accusations of monopolizing platforms and whatnot, but I don't have the links with me now.
So at this point, the fan communities are pissed because there's no more shmexy fanart, the regular artist communities are pissed because there's no more figure drawings or paintings of ladies and urns, the SJW sects are pissed because this flies in the face of all that sex positivity discourse and the fear that "adult content" may become synonymous with "queer content," thus striking a blow towards LGBTQ user expression, and the horny people are pissed because no more porn.
Whether or not this decision will be rolled back, if the userbase will migrate or remain, or if Tumblr will go the way of Livejournal is yet to be seen.
In the words of Gravity Falls creator Alex Hirsch (whose commentary has been hilarious), Congrats to Tumblr for finally destroying its arch enemy, Tumblr.