Warranty and support specifically for that flag? Because I don't see how general warranty and support requires keeping any hint flags forever.

If you remove the hint flag peoples build will break

Doesn't need to, it can acknowledge and ignore the hints.

True, but there might be more problems — like if you drop support their run time will be slow because they rely on this flag and they are unhappy

The premise of removing the flag is that it's useless or a problem. If it's still causing a big speed boost somewhere then you need to figure something out, but the core scenario here is that it's obsolete.