Wish I was there to witness it tbh it's sad what 4chan has become
They were failing miserably at removing child porn from their site, and decided that banning all the porn would somehow help. It probably won't, their algorithm is wack. Rather than moderating their platform it seems they preferred the nuke option.
In particular, users have pointed out some issues with Tumblr's approach to removing child porn, like the fact that there's nothing on their help site to tell users what to do if they find it, and the only way to report such material is to click the 'share' button.
Seriously, to report child porn on Tumblr you have to click 'share'.
Tumblr management is incompetent.
When they remove child porn images, they tend to leave the original posts and their accompanied notes intact. So, basically, they're leaving up lots of these "Hey, we removed illegal material that was here, but please click below to see 200 more blogs that might have what you're interested in". They are essentially accomplices to distribution of child porn.
The child porn pictures often get reblogged over a thousand times before they are removed, which can take a week or even months. There are a lot of child porn images that have been on the platform for several months, even though numerous users (but not all of them!) reblogging the illegal image have been banned. Even worse, banned users just recreate their accounts, sometimes incrementing a number in end of the account name. Those numbers eventually reach double digits, they just keep coming back.
There are also "beacon" images that don't get removed at all, since they only contain text in the image such as "reblog if you want to have sex with children". Using these, pedophiles can easily find hundreds of other pedophile blogs and reconnect back to the grid after they are forced to recreate their account. Some of the beacons have existed for several years.
The pedophiles on tumblr are fearless. There are people taking selfies of themselves, showing their face, and then showing them jacking off to child porn that's on their computer screen. There are people openly trading their phone numbers to connect in Whatsapp to trade images there, and although the international numbers I've seen have mostly been China, Vietnam, Mexico, Brazil, and other similar places a bit further away from the western civilization, there are users from Europe and USA posting their phonenumbers on those beacon posts. There are posts about "pedo pride" and the whole thing is outright crazy. There are thousands of active pedophiles on tumblr. THOUSANDS.
And what does Tumblr administration do? They nuke all the NSFW. I'm not entirely sure whether this will truly eradicate the child porn from Tumblr, considering the nature of the tumblr pedophile community. We'll see.
What they should've done? First of all, they should've made reporting cleaner. There's no obvious way to find the reporting functionality at all if all you do on tumblr is following others. Their help center has zero results for "child porn" or "illegal", and when you finally find the help page for reporting content, it informs you that
you need to click the "SHARE" button on the child porn images to report illegal content. Not kidding. Who the hell designed this system? I don't know about you, but I'm sure as hell NOT clicking SHARE on any child porn image, even when I know it opens a popup where I can choose to report it.
Second, they should delete deeper. Most importantly, they should delete the posts when they delete the associated images. Pedophiles are delighted to see empty frames of pictures that have been removed for violating rules, because it allows them to find new blogs that reblogged the illegal content in question. Many of those blogs are still going to be completely intact. It's also not uncommon to see child porn images with several thousand notes and almost everyone involved with it banned, but the picture intact. Seriously, if half of the users reblogging a certain image are already banned, maybe they need to review that image to see if it has to be deleted.
Third, they should ban people who hit "like" on illegal images. A large number of pedophiles do not reblog anything, they instead hit the like-button which keeps their content a bit more "stealth" from the masses. Other pedophiles can still find the material by browsing the liked-by lists. Apparently accounts that like illegal content won't get banned that easily, either, which is utterly ridiculous.
There are many other things they could do, from verifying users with SMS to building better tools for exploring the site (to bring the things that hide in darkness into the light). All of the effective measures, however, require quite a bunch of work. Banning NSFW is just a policy change and isn't expensive to enforce, although it might be expensive in terms of users lost.