It definitely affects current indexes. We can't completely avoid bad user functions. That is why it is important to put limits on how much damage they can do. That's the motivation for the idea I mentioned before, of double-checking visibility data in an IndexTuple before letting it survive a VACUUM.
No i don't say it would affect Vacuum, but i am suspecting that it would affect Index based select. Since Vacuum uses a sequential scan of tuples, it doesn't require the ordering operator, but any index based search would require a ordering operator for binary search and for comparing with the right most key.
Thanks,
Gokul.