As I understand it (as a light Tumblr user, in areas adjacent to but almost entirely outside of the NSFW sections), a significant number of the porn blogs have, in fact, been deleted, as well as a significant number of blogs or posts on blogs that were flagged as porn by their egregiously terrible porn-finding algorithm, but were in fact nothing of the kind.

However, the porn spambots still keep popping up periodically, and there are still loads of Nazi/alt-right blogs that Tumblr doesn't seem to have any intention of doing anything about.