That's a very reasonable response. I'd add another thought: A site (e.g., Twitter) could define "off-topic" as anything that goes against its Terms of Service o

That's a very reasonable response. I'd add another thought: A site (e.g., Twitter) could define "off-topic" as anything that goes against its Terms of Service or Code of Conduct (CoC), which would probably result in exactly where we are now. Same problem with a lot of similar ideas - "Spam? Anything that violates our CoC is spam"; "Degrades the experience for the user? We feel that violating our CoC degrades the user experience"; and so on. Not a problem easily (or even able to be, IMO) solved.