So, with this hypothetical attribute, the thing isn't gone, but it doesn't compile any more, whereas with the current situation the thing also isn't gone, and you get a warning (unless you told the compiler to forbid this rather than warning, in which case it doesn't compile).

This is identical to a [really_deprecated] attribute. We know it's deprecated but kelnos wants to force us to use a separate crate to use it for some reason.