Apparently they had already decided to do it before the App Store thing happened.

https://www.vox.com/the-goods/2018/12/4/18126112/tumblr-porn...

> But a former staff engineer, who recently left Tumblr and asked to remain anonymous for professional reasons, tells Vox that the NSFW ban was “in the works for about six months as an official project,” adding that it was given additional resources and named “Project X” in September, shortly before it was announced to the rest of the company at an all-hands meeting. “[The NSFW ban] was going to happen anyway,” the former engineer told me. “Verizon pushed it out the door after the child pornography thing and made the deadline sooner,” but the real problem was always that Verizon couldn’t sell ads next to porn.