Right now, Plasma doesn't respect the system scale factor on X, only Wayland. However there is actually support; you just need to run plasmashell with PLASMA_USE_QT_SCALING=1.
There were two major blockers that have both been resolved upstream of us:
- Ugly text with fractional scaling in Plasma itself, fixed in KDE Frameworks 5.45: D11244
- Ugly text with fractional scaling in various system settings places, fixed in Qt 5.11: https://codereview.qt-project.org/#/c/222827/
If we ship with Frameworks 5.45 and take the trivial, two-line patch from https://codereview.qt-project.org/#/c/222827/ into whatever Qt version we ship with, then we can turn on Qt scaling in plasmashell and fix a significant amount of bugs with running Plasma with a HiDPI screen.
To turn this on, we would patch /etc/xdg/autostart/org.kde.plasmashell.desktop to replace plasmashell with PLASMA_USE_QT_SCALING=1 plasmashell.