After every update (possibly even more frequently) KDE Linux creates a hidden folder in the root directory, approx. 6 M each, cluttering my system partition. Is it safe to remove old folders (and which ones)?
It’s the same on my system. Maybe it will be used in the future for rolling back /etc.
Correct! Those are btrfs snapshots of /etc that will indeed be used for the automatic rollback feature.
Why do we need hundreds of these snapshots if the update system keeps only three previous images?
The general reason why you would want more snapshots of any mutable data, I suppose: more opportunities for recovery if you need to restore from a working setup.