I don't think you can viably enforce strict sourcing requirements at an instance-wide level because, like you mention, most posts fall into one of two camps, either it's OC or something that someone found and saved a decade ago. People can do their best with reverse image searching, but for a lot of porn content it will have been reposted to oblivion already and that may not be reliable. The fact that anyone can make a new community at any time just makes things even harder.
I agree that not listing sources when you know them is at best rude and at worst unacceptable. Not to call anyone out(although they are a FREQUENT offender), but I just saw this post a bit ago.
https://lemmynsfw.com/post/28978835
This is literally a user of this site who posts content for free as advertising and you can't even be bothered to credit them or cross-post? Seriously?