The Social Network giveth, and The Social Network taketh away.
After recently announcing a new multi-contributor Stories option for public figures, Facebook has followed that up with the removal of its similar 'Group Stories' functionality, which enables Facebook group members to contribute to a single, collaborative Story within that group.

And they're not giving much notice either - as explained by Facebook:
"On September 26th, we will be sunsetting group stories. At that point, no new group stories can be posted and existing group stories will be deleted. Members and admins can continue to use stories from their personal profile or make a post in the group to share moments from their lives."
That'd be Thursday next week.
Facebook launched the first iteration of group Stories to selected users in November 2017, before expanding the functionality to all groups last December. But while group Stories added a new consideration for group engagement, it also added extra moderation workload for group admins. Last October, ahead of the full rollout, Facebook added the ability for admins to mute or block members in order to stop them from contributing Stories content which was unrelated to the group. Facebook also added the capacity for group admins to delete or report any story photos and videos which they felt were inappropriate.
It seems that the extra management required hasn't matched the usage benefit, and Facebook's not wasting any time in removing the option entirely. Which could also suggest that there is another, more pressing reason for their removal, but we don't have further insight at this stage.
Facebook says that archived group Stories will remain viewable, but users will not be able to re-share them. Facebook also notes that any pending Stories admins have queued will also be deleted next week.
On one hand, you could look at this as a signal that maybe Stories aren't as immersive and/or popular as Facebook has been telling us they are, repeatedly, for the past two years, but then again, the stats show that Stories use, overall, is on the rise. And this is only one element, which, given the capacity for various people to add content, seems open to potential issues, particularly without active moderation.
So, it's time to bid farewell to group Stories. As of next Thursday, they will be no more. Adjust your group strategies accordingly.