KIO stopped reacting to this env var with KF 5.88.

See https://invent.kde.org/frameworks/kio/-/merge_requests/599

Any projects requiring KF >= 5.88 thus can stop setting this variable.

Remaining users by lxr.kde.org:
https://lxr.kde.org/search?%21v=kf5-qt5&_filestring=&_string=KDE_FORK_SLAVES&_casesensitive=1

Initial clean-up for tests in KIO: https://invent.kde.org/frameworks/kio/-/merge_requests/1048