Here's the announcement on the Yahoo Groups site:
Attention: Starting December 14, 2019 Yahoo Groups will no longer host user created content on its sites. New content can no longer be uploaded after October 28, 2019. Sending/Receiving email functionality is not going away, you can continue to communicate via any email client with your group members.
As I understand it, the groups will continue to exist and act as a group email discussion service. However, all the storage of files, photos, etc, is being eliminated.