While this may well be a factor in their decision, it seems highly unlikely to me that (a) Apple was entirely unaware porn existed on Tumblr for the years the app was in the store before November 2018, and (b) the only way Tumblr could ensure there was no child porn on their servers was to ban everything that smacks of NSFW-ness down to "female-presenting nipples." Tumblr's CEO said they've been considering this move for months, and I don't see any reason to doubt him.

Tumblr has never made money, as far as I know, and Yahoo's new corporate parent Oath seems to be a lot less mellow about that sort of thing. I suspect this is a necessary step before they open the advertising floodgates and see if they can get Tumblr to start paying for itself. Losing a massive amount of their userbase is an acceptable outcome as long as there's still enough users there for more advertising to turn a a profit.