I’m running into same issue, I use no widgets or plasmoids, just the default systray applets that came with kde 6.3.2.
I have a 24/7 on pc (no nvidia card, igpu only) that runs some docker containers and vms and occasionally use it as media pc in the evening. This morning plasmashell was using 9GB memory causing heavy swapping and I had to reboot. This was after 2 days of just sitting, running tasks in background.
I observe the same on my laptop (has discrete nvidia), but that reboots more often so is not much of an issue.
Both use arch and kde 6.3.2 with just the default systray applets and icon only taskbar, no plasmoids, no widgets.
I observe this behaviour since about ~3 weeks ago (so almost aligned with when OP experienced it) It must have been brought up by some update as I update both machines at the same time from the same mirror.
Any help on how to troubleshoot further? What debug info can I gather next time I see the high memory usage?