Microsoft cut off the ability to create new Groups in OneDrive in 2014, along with a way to add new users to existing Groups. Now the company is informing users via email that all existing Groups will be inaccessible to all OneDrive users on October 16, and that data in those Groups will be deleted.
The slow death of the Groups feature still allows people who have them in their OneDrive account to download them and move them to their main account before the deadline. The procedure can be checked out at Microsoft's support page (which lists an incorrect September 30 date for the Groups shutdown).
Thanks to Prabu for the tip!