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=true`.
This is not enabled by default because Plasma developers [[https://bugs.kde.org/show_bug.cgi?id=356446#c22|feel that it's not ready for prime time yet|]]. But it [[https://bugs.kde.org/buglist.cgi?bug_id=366088%2C372013%2C382146%2C389813%2C391682&list_id=1502394|fixes a significant amount of bugs]], and the font kerning issues it (only supposedly) causes seem to be minimal or non-existent, especially on newer Qt versions.
In order to better support our X11 HiDPI users, I propose that we patch `/etc/xdg/autostart/org.kde.plasmashell.desktop` to replace `plasmashell` with `PLASMA_USE_QT_SCALING=true plasmashell`.