Again it depends on what those repositories do.
EndeavourOS (Arch based) adds a repository which appears to be for their utilities, otherwise they use the Arch repositories. You could probably continue using it with minimal disruption although the utilities would be unmaintained.
Manjaro (also Arch based) uses its own mirrors of the Arch repositories and adds some of its own. If it vanished, it would quickly become out of date and full of security holes. A new install would be necessary.
In either case, I’d do a clean install of Arch because why give yourself the headache.
Good camouflage.