A lot of people will blame Yahoo for Tumblr's demise, but Tumblr's product just did not do enough to keep up in the mobile era. They were soundly out-competed for attention by Instagram, Pinterest, Reddit, and Facebook.
It was a poor experience lacking in engagement. Using Instagram and Reddit was much more fulfilling in creative and online social/community needs.
Tumblr's numbers were flat lining before they removed porn. I know this because my friend was a Tumblr engineer and would share data from their all hands meetings.
FOSTA became law in April 2018.
So please explain to us how FOSTA was responsible for Tumblr's demise.