PoppaGator wrote

To me, "unaccompanied" singing would denote solo vocal-only performances, and might even be understood to include self-accompanied performances (e.g., guitar and vocal, as practiced by many of us).

Most of the music under discussion here involves group singing without instrumental accompaniment, where the singers are, essentially, accompanying each other. Hence, as far as I'm concerned, "a cap(p)ella" is a more accurate term than "unacompanied," even when the music is not "church style," regardless of the term's ancient etymology and its literal translation from the Latin (or Italian).

I think this is more a difference in terminology on the two sides of the Atlantic. In the UK the term normally used is "unaccompanied" both for solo and group singing. "A capella" tends to be restricted to classical singing groups. In fact I have even heard the term "unaccompanied" used for classical groups.

To add to the list on the thread, no one has mentioned the Keelers from Tyneside. Excellent unaccompanied harmony singing.