Just because you can't find it doesn't mean it's gone. There are people out there other than the major search engines that scrape websites, and some of them don't post their collections online at all. Especially for things like nude pictures, there's a much higher chance that people will find and keep that stuff.
Obviously, not literally everything is permanent, but the point is that you can't possibly know for sure that it's gone. Posts that have been gone for years can always come back.
Just because you can't find it doesn't mean it's gone. There are people out there other than the major search engines that scrape websites, and some of them don't post their collections online at all. Especially for things like nude pictures, there's a much higher chance that people will find and keep that stuff.
Obviously, not literally everything is permanent, but the point is that you can't possibly know for sure that it's gone. Posts that have been gone for years can always come back.