When I started performing (in Oz), groups (pairs, trios etc up to about octets) that sang harmonies with no instrumental involvement were always (in the folk scene) described as singing unaccompanied harmonies; soloists that did so sang "unaccompanied" but not with harmonies unless the audience got into the act, which happened. The fashion changed in (as far as I can tell) in the 90s, when commentators with more classical training than knowledge of the folk scene started using "a capella" (and a cappella) to describe such items. Of course, it may have been that such commentators were using the term for the obligatory non-instrumental item in any Steeleye Span concert a couple of decades earlier but there weren't many such commentators on radio/tv in Oz at that time so I wouldn't have known.

Cheers, Rowan