There are many threads on the forum about this subject already, so please, next time perform a search. Here’s a still fairly recent example, albeit perhaps not the most recent one. 
You can start by first removing all of the packages in that list that are shown as “alien” (
) — those are all either AUR packages or packages no longer featured in the repositories. I can already identify a couple of them from here without having to look up on them — e.g. ceph-libs, which was dropped to the AUR and doesn’t even build properly anymore if you try installing it from there.
Other than that, some of the qt5-specific stuff can also safely be removed because it was already superseded by a qt6 version. Examples below…
kactivities-stats5kdesu5kdnssd5kfilemetadata5krunner5ktexteditor5
There are plenty more — and not just qt5 stuff — but you should check the others ending in 5 or -qt5 on whether there already are qt6-based replacements for them in the repos. If it’s an orphan from the AUR, then by definition it’s not needed — it may even have only been a build-time dependency for something else.
When in doubt, check with your package manager.
Oh yes, you can. 
If your initial installation is old enough, then the newer versions of the packages from the original install medium may still be marked as “explicitly installed”, rather than “installed as a dependency”.