Nevertheless, copyright can subsist in an arrangement of a public domain song. Simply don't copy somebody else's arrangement, if you're worried about it; and bear in mind that if you learn a song from a commercial recording there will be existing rights to that form of it either claimed or actual.

In practice it matters not at all unless you intend to record the song. Beware of people who make sweeping statements such as "Scarborough Fair is public domain and any claim otherwise is bogus"; that would depend on which of a great many different forms of the song sometimes called Scarborough Fair is being talked about. Many are in copyright, like it or not; but that is a very complex issue and need not concern the amateur performer.