It would be nice if we were able to mark a variable that is set as deprecated so that the consumer of that variable is made aware he should not depend on it.
Something like
set(Foo_OLD_AND_LAME ON DEPRECATED "Use Foo_NEW_AND_RAD instead!")
This came up a few times with variables we were exporting and we ended up guarding the definition of these variables
if(Foo_ENABLE_DEPRECATED_STUFF)
set(Foo_OLD_AND_LAME ON)
endif()
which works but it clearly suboptimal.