In May 2020, Google announced new Google Groups to provide an updated experience that matches Gmail and other Google services. In September 2020, Google made new Groups the default experience for all users, but they gave users the option to use classic Groups instead of the new interface.
According to Google, starting on November 16, 2020, classic Groups will be turned off for everyone. This means that after November 16, there will be no option to change back to classic Groups. If you have already been using new Groups, you will not notice a change, but if you have chosen to keep using classic Groups, all remaining classic Groups will be automatically transitioned to the new interface.
A few features are not yet available in new Groups, including welcome messages, global settings, and custom group roles. There is currently no timeline on when they will be available.
In addition, several features will be going away, such as the ability to edit your posts. To see more features that are going away with classic Groups and new features available in new Groups, see Google’s What's new in Groups.