kdeinit5, kscreen_backend_launcher, kwin_x11, xembedsniproxy, and a bunch of other programs keep writing massive amounts of verbose messages into the syslog, making it pretty much impossible to find any meaningful messages in there.
On kde4, these messages were all logged on stderr, which means they ended up in the user's .xsession-errors instead of the global system log. IMO, the user's homedir is where they belong, since all those messages are only ever relevant to the user running the plasma/kde session, and moreover, most systems I have worked with do not grant access to system logs to regular users.
The redirecting from stderr to syslog is done by systemd, I don't think it's anything we are doing.
qDebug just outputs to stderr.
But you can use kdebugsettings to turn it all off.
While kdebugsettings can disable _some_ logging (maybe even most of it by size which will alleviate this particular issue) there are still messages that cannot be toggled after compilation, I've created new report for that #409521
I can confirm that plasmashell 5.25.3 fills the systemlog with output from the programs it launches, although QT_LOGGING_RULES='*=false', all log output is disabled in kdebugdialog and almost all debug output is disabled in kdebugsettings.
I think that is nor really avoidable, you need to report the issue to the apps that output stuff. e.g. for Kate I tried to minimize that.