Through a quick and highly useful response to a specific question viewtopic.php?f=67&t=141961, I became aware that my system probably contains lots of "legacy stuff" that is no longer needed. I know that a way to a cleaner system could be a completely fresh reinstall, but I prefer to improve my running system.

Where can I find guidance? Can I, for example, in a completely updated qt5, plasma5 etc system on opensuse leap 42.3 begin to uninstall everything that has kde4 in its name?

That sounds risky to me, but still I wonder whether any guidelines exist for a way to free the system from old stuff that is no longer necessary.

I'd suggest asking in OpenSuse's forums as they may be able to point you to tools available for OpenSuse for cleaning up things like obsolete/legacy packages.

You might be able to uninstall most things with kde4 in the name (especially if the version number indicates a version more than a couple years old). I believe there are still some straggling applications which haven't switched over to kf5 yet, but most of the base apps have. Also if you have any apps with both a 4 and 5 version then you can certainly remove the 4 version (you can run them side-by-side without problems since they read configs from different places). (again, asking in OpenSuse's forums would give you a better answer; though there are a few regulars to this forum who use OpenSuse and may also be able to provide a better answer)

Thanks, I made an attempt with yast2 and was able to remove a number of things (kde4-filesystems, for example), sometimes yast2 suggested additional packages to delete along the way. This ran smoothly.

However, I thought it would be safe to make a "zypper dup" just before rebooting, and zypper decided to put everything back where it was.

I do not know if my system would have worked without the zypper after a reboot, but I don't want to take too big risks either.

If it's mainly about removing obsolete KDE Plasma 4 components, I guess the easiest way would be to temporarily install another DE (actually a WM would suffice) and simply completely uninstall KDE.

Then install the current KDE version, and you shouldn't have to worry about outdated Plasma components anymore.

To me this seems easier than determining the current utilisation of dozens of Plasma components, one by one.

Cheers,
Gaius