I don't have a ton of faith in any legislature's ability to usefully carve out what delegation and APIs should be mandated, but portability could do a lot... if it were required that you be able to import data you'd exported/deleted, even if you'd modified it. Deleted comments wouldn't have to show up to other people back where they were, modified data could be marked as such, and reasonable exceptions could be made... But then you could build, say, a third-party interface for operating on one's own data with the modified data as a result. And I hear that the GDPR data exports change format pretty capriciously--if there were requirements about being able to import as well, that'd be fixed right quick.

This is definitely one of those "engineer thinks their 'one clever trick' will solve complex social problem" ideas, but I think it'd at least help.