This was added as an attempt to fix double scaling on high DPI specifically on Plasma. This is better addressed by b31852c4caa36cc564e25adbdacfa534e1dfe7c0 in combination with https://phabricator.kde.org/D26185
Unfortunately it provides a problematic behavioural change within a patch release.
We expose xrdb values in our settings, both explicitly and implicitly the scale factor to allow fine grained font control. This is particularly important for our window manager which can't easily use Qt scaling as it conflicts with it's own scaling, but we want appropriate fonts.
We also use xrdb to provide a specific font size to xwayland apps which is still relevant for SuSE.
(I would support this patch if it was guarded by an env variable instead of Qt trying to detect plasma)