As reported in MBS-13440, the API currently accepts the prefixes sa- (presumably for Single Artist) and va- (presumably for Various Artists) to the release status and the release group (primary and secondary) types.

The code changes can be traced through the commits fa5fe244, 48268b75 and 75d597e4 in connection with MBS-386, that is for backward compatibility with ws/1.

Dropping this undocumented legacy support is a minor but still breaking change that should be conveyed through ws/3.