I have purchased 10-ish CD box sets of Paul Simon and Joni Mitchell but these are just repackaged previous albums in a convenient box, so perhaps do not count (no new material/retrospective view). I think there is a 30 CD Elvis set and a few for groups (Queen, Wishbone Ash), and as you say, maybe others in the jazz world (Charlie Parker, Sun Ra). So, maybe the Sandy Wikipedia article could say "unprecedented in the folk world" or similar?