This change will be incompatible. -1, needs a better design

oleg_nenashev How about providing both vars for a while with at task for removing the "slave" one in six months? Is eternal backward compat required?

The originally agreement was that the term slave would be removed from the UI, but left in the APIs to avoid breaking compatibility both for plugins and customer tools/scripts.   The problem is that line of where the UI stops and the API begins is a somewhat fluid when your users are engineers.

I understand that env vars are effectively part of the API and so changing them is problematic and could be considered out of scope.   But they are also part of the end-user experience (the UI) for every Jenkins user.