To be clear, -Qm displays installed packages not currently in the repositories. This will include AUR packages, but I avoid the AUR (except for davmail years ago) every once in a while I’ll run it just to check and sometimes it finds packages.
When you install things from the main repos the dependencies get installed too, and if those dependencies are no longer needed they’ll be removed from the repositories. (I also have a bad habit of forgetting --asdeps when installing optional dependencies.) Sometimes they’ll conflict with a new dependency and pacman will ask to remove and replace them, but other times the functionality has become a part of an existing package, so with no conflict to prompt removal they’ll just sit unused on your install. If you haven’t tried -Qm in a long while you’ll probably find a few harmless currently-unused packages that were installed through the normal repos. (-Qdt will cover the other cases where dependencies remain in the repos but are now only needed for packages you don’t have installed.)
Obviously -Qm will also show removed packages that aren’t dependencies, a few years back my preferred pdf viewer was removed from the repositories.
-Qm will also find manually installed packages that aren’t in the AUR if you ever do that.
I mostly agree with this. The reason I stick with arch has nothing to do with finding it technically superior or more convenient to use than other distros. There is almost no way I would be using arch if their wiki wasn’t leaps and bounds more helpful than other distros’ documentation. Some other distros can meet my needs just as well or better, but arch’s documentation allows for a more straightforward learning process through tinkering.
I do think repository differences can potentially matter a lot. I have a lot of respect for hyperbola being ultra-hardline in removing proprietary packages and any hint of uncertainty in licensing, but the fact that that kills off texlive makes it untenable for me to use. Keeping manual installs up-to-date is a hassle, so I can definitely understand someone limiting themselves to distros supporting a certain set of packages.