Tumblr has updated its Community Guidelines so that "adult content" will no longer be allowed on the platform as of December 17, 2018.  Tumblr defines "adult content" as content depicting "real-life human genitals" or "female-presenting nipples," as well as any content that depicts sex acts.

Tumblr is allowing its users two weeks to remove prohibited content.  During this period, Tumblr will be using algorithms to flag content as adult content.  After the notice period, the flagged content will be removed. 

Tumblr has also instituted a process whereby users may appeal Tumblr's flagging of a user's content as adult content.

More information on this update to Tumblr's Community Guidelines may be found here