Can confirm it’s for GroupsV2, but I did not know they were mandatory in official Signal now. That moves the up the priority list. Sorry for the inconvenience!
EDIT: actually, there’s still a difference between creating a group with a WF user, and adding a WF user to a group. The former is still possible, the latter is not. Please do track https://gitlab.com/rubdos/whisperfish/-/issues/86, `beta.1. will include Groups v2!