A net journal called "The Verge" broke the story, but I saw it on Twitter, which probably ciculcated the story more. Anyhow, Tumblr has announced that, starting on December 17, "adult content" will not be allowed on Tumblr.  Tumblr posted this item itself:

Adult content

Is adult content allowed on Tumblr?
Starting Dec 17, adult content will not be allowed on Tumblr, regardless of how old you are. You can read more about what kinds of content are not allowed on Tumblr in our Community Guidelines. If you spot a post that you don’t think belongs on Tumblr, period, you can report it: From the dashboard or in search results, tap or click the share menu (paper airplane) at the bottom of the post, and hit "Report."
What is "adult content?"
Adult content primarily includes photos, videos, or GIFs that show real-life human genitals or female-presenting nipples, and any content—including photos, videos, GIFs and illustrations—that depicts sex acts.
What is still permitted?
Examples of exceptions that are still permitted are exposed female-presenting nipples in connection with breastfeeding, birth or after-birth moments, and health-related situations, such as post-mastectomy or gender confirmation surgery. Written content such as erotica, nudity related to political or newsworthy speech, and nudity found in art, such as sculptures and illustrations, are also stuff that can be freely posted on Tumblr.
What will happen to my adult content already on Tumblr?
We will send out email notices to members of the Tumblr community whose content has been flagged as adult. If your post has been flagged as adult, it will be reverted to a private setting viewable only by you. If you want to learn more about how to see those posts, see this help article.
As always, please make sure the email associated with your Tumblr account is one you use regularly. It’s how we get in touch when we need you!
My content was flagged as adult, but I don't think it should be. What should I do?
If you feel that we have categorized your post incorrectly, you can appeal this decision using the button on the post in question. Please note that this process is only possible to complete on the web or the Tumblr Android app version 12.2 or later to review your flagged content. If you are on iOS, please use the web to appeal for now.
Read more about how to review your content and appeal here.
What if my blog (not to be confused with posts) was marked as “explicit” before December 17, 2018?
Blogs that have been either self-flagged or flagged by us as “explicit” per our old policy and before December 17, 2018 will still be overlaid with a content filter when viewing these blogs directly. While some of the content on these blogs may now be in violation of our policies and will be actioned accordingly, the blog owners may choose to post content that is within our policies in the future, so we’d like to provide that option. Users under 18 will still not be allowed to click through to see the content of these blogs. The avatars and headers for these blogs will also be reverted to the default settings. Additionally, posts from these blogs are kept out of search results.
You can check and see if your blog is marked as explicit per our old policy in your visibility settings. If you think your blog has been erroneously marked as explicit, learn how to appeal here.

A couple of comments.

First of all, way overdue, and I wish others would follow.

I don't know much about Tumblr.  Rather obviously, I use the Blogger format for this blog.  Tumblr is a popular blog format but it's also basically a dumpster fire in my view. I.e., it's quality is low.

Therefore I know very little about Tumblr, but while it is popular it apparently had a reputation for hosing a huge amount of pornography.  And apparently it got busted in some fashion, i.e., warned, t hat some of that stuff was illegal.  That's apparently what motivated Tumblr to do this.  I.e, it wasn't suddenly self motivated by become aware that it was acting immorally and to the lasting detriment of,  primarily, women.

It doesn't appear to be getting completely out of the porno business, however. And as a place hosting pornography, it's in that business.  Make no mistake about it, as the host, it's promoting pornography which in turn promotes and supports everything that pornography does.   And what that is, is prostitution, white slavery, drug use, and a host of other icky crap.

But here's another observation, or perhaps a comment, or a question.

Why has pornography acquired the moniker "adult content"?

Adult? There's nothing adult about it.

Focusing on nude young women for lustful viewing pleasure by men isn't adult, or at least it sure isn't mature.  And the whole society should take a lesson about that.  Indeed, you'd think that in the "Me Too" movement era, people would clue in that this stuff promotes the very "all young women are sterile bimbos" message that got all that rolling.  We've discussed the history of this before, but adult? 

No, I don't think so.  It's not juvenile, to be sure.  But it sure isn't adult.