I think having a publicly available history link that displays every TOS and Privacy Policy and the dates they were inforce isn't too much to ask. Maybe a last ten years thing is applicable (not retroactive). Users don't keep copies of the TOS they agreed to at the time of signup/registration as they expect it to be available from the service/corp/org.

I think asking for a visible diff is asking for trouble, but being able to manually see how their TOS changed over time and how often it changes would be an objective way to ensure customers can make informed decisions, if they wanted to take the time.

Edit: Available on request is a usability anti-pattern to deter consumption of information. I don't think it would be outrageous to require previously public legal documents to remain publicly available.

If the government got into the habit of regulating conveniences back in the 1970s, we'd have ("Contracts must be typed with this type of ink, and signed with this type of ink so that it scans well", "Contracts must use this size paper to be easily scannable") and the entire business of e-signing documents would've never happened because there'd be so many legacy usability requirements.

You'd be faxing forms to hip new startups to agree to their terms, signing your name next to every major paragraph, until somebody successfully pushes a new form of contract signing through the government.