I don't immediately see how this will work with index-only scans. If the tuple is HOT updated several times, HOT-pruned back to a single version, and then the page is all-visible, the index entries are guaranteed to agree with the remaining tuple, so it's fine to believe the data in the index tuple. But with WARM, that would no longer be true, unless you have some trick for that...
Well the trick is to not allow index-only scans on such pages by not marking them all-visible. That's why when a tuple is WARM updated, we carry that information in the subsequent versions even when later updates are HOT updates. The chain conversion algorithm will handle this by clearing those bits and thus allowing index-only scans again.
