On Monday, December 3rd, 2018 (12/3/2018) the CEO of Tumblr Jeff D’Onofrio made a post regarding the adult content (18+/porn/nudity) on the website and as of December 17th, 2018 ALL these content are going to be PERMANENTLY BANNED from the platform itself!
Yes, this includes any and all adult fanfic images/videos or anything pornographic itself!
Source: A Better More Positive Tumblr