I'm going to comment only on this, but the others (most, if not all) are the same. If you check the definition of _define_relative, this parameter is:

oldstylename (optional): the old-style name of the variable

So this command is exactly defining the old variable in addition to the new one.
We can't remove the definition of the old variables until they are in use, and for sure not during the lifetime of Frameworks 5.x.

Unless of course I misunderstood the code...

Hmm, you're right, but then it shouldn't come up with a warning...?

In the _define_relative macro we have:

if(KDE_INSTALL_${varname})
    ...
elseif(${_oldstylename})
    if(NOT CMAKE_VERSION VERSION_LESS 3.0.0)
        message(DEPRECATION "${_oldstylename} is deprecated, use KDE_INSTALL_${varname} instead.")

In my case the message was triggered for all definitions.