@mlaurent Why the switch to the old unprefixed (and deprecated) variable here?

Oops indeed I made an error I read "SYSTEMDUSERUNITDIR Systemd User Units (lib/systemd/user) [SYSTEMD_USER_UNIT_INSTALL_DIR]. Since 5.65 " for me it was the last version of variable.

Perhaps we need to make it as deprecated.

mentioned in commit 4294f03c

I would not know a way to mark cmake variables as deprecated, so putting things into the docs is all we might be able to. Currently there is a general comment at the top of the list of vars in the docs, saying

default values in parentheses and alternative, deprecated variable name in square brackets)

Repeating a "deprecated:" text in all places might be too much noise, also will blow up the layout in some places.

Any ideas how to improve things surely welcome.

Though it is strange that at 5.65, when the unprefixed ones were already deprecated, this new one was added. No idea why, in any case, gone in KDEInstallDirs6.