Setting KDEWM to something other than KWin no longer works in Fedora as of 35 (and other "leading edge" Plasma distros like OpenSUSE are quickly approaching this turning point as well). There are several spots in the Plasma code base that KWin is hard coded. Those distros that still allow functional KDEWM only do so because their version of Plasma is either not yet new enough to be affected, or they have deliberately held back parts of Plasma (which means at some point they'll have it give in as well).