Mixed DPI multi-monitor setups are not officially supported on X11, but they can be bodged into working. If you set PLASMA_USE_QT_SCALING=1 to enable Qt scaling on X11 though, everything goes haywire and nothing works properly at all. See https://invent.kde.org/plasma/plasma-workspace/-/merge_requests/139#note_69756.
This must be resolved if Bug 356446 is going to be implemented.
Opening this purely so it can be marked as a blocker for Bug 356446, for tracking and recordkeeping purposes.
The thing this is a blocker for (Bug 356446) is closed now, and doing this was never supported anyway.
Unfortunately I think we have to close this. Folks who want a decent hidpi experience will have to use Wayland and look forward to upcoming changes to improve the experience for XWayland apps in particular.
Sorry everyone. :(