KWin seems to have a memory leak. It will steadily increase memory usage over time, usually reaching 150-175mb within a day.
The memory never gets released. On long uptime cycles I've seen KWin rise to over 800mb memory usage. This is all private memory.

There's one very clear contender I've noticed here: Using 'Present windows' or 'Present desktops' will increment the RAM use consistently
I can open up ksysguard, look at kwin, and press the hotkey multiple times, and every single time, without exception, the memory usage increases.
It does not go back down afterwards.

I usually run 9-12 virtual desktops, have at least 20 windows open, and use the present windows/desktop functions a lot, which is probably why this becomes so noticeable. Is there a setting that keeps 'present' thumbnails and forgets to delete them or something? My current solution is to just do 'kwin --replace' on a timer. Works, but not clean.

This has been an issue for me through several plasma/kwin/kde versions by now. At least 3 different system upgrades on Kubuntu and a Neon install (currently on Kubuntu) as I recall. plasmashell used to rise more or less in tandem with kwin, but this seems to have gotten better with later versions and as far as I can tell it now stays below 120mb.
I've tried all the different rendering backends for the compositor, all with the same results.

Graphics: Mesa Intel HD Graphics 5300 (BDW GT2) (i915 kernel module)
OpenGL ver.: 4.6 (compatibility profile) Mesa 21.2.6
Plasma ver.: 5.22.5
KDE Framework ver.: 5.86.0
Qt ver.: 5.15.2
OS: Kubuntu 21.10

I suggest you upgrade your system to Kubuntu 22.04 and check if this still happens. If yes, please file a bug report with your findings to
https://bugs.kde.org/enter_bug.cgi?prod ... nt=general