That is not true at all. All Tumblr blogs, including NSFW blogs, have always been viewable and indexable by external search engines through their public URL (blogname.tumblr.com) unless flagged as private/hidden (a choice of the blog owner). They have only been hidden from Tumblr's search engine if the user has their account set to not see adult content. You can verify this by searching for "tumblr porn" on Google. Five of the results on the first page are porn blogs on Tumblr.
>All Tumblr blogs, including NSFW blogs, have always been viewable and indexable by external search engines through their public URL (blogname.tumblr.com) unless flagged as private/hidden (a choice of the blog owner).
There is definitely something in account settings somewhere that removes you from search results, I may have mixed that up with the NSFW flag.
>They have only been hidden from Tumblr's search engine if the user has their account set to not see adult content
As I mentioned, this is default. And "adult" blogs do not show up at all in search results on mobile.
My point stands, though. Yahoo already started making adult content harder to find (accidentally or intentionally), the momentum is already there.