The MMS help does not contain an MMS group upgrade feature list (or compatible features) to list features that are being improved added vs features that are being deprecated / removed.

This results in a ticket on a periodic basis with MMS dashboard users who upgrade to the new groups.

Not sure if there any such other feature as well which was removed but haven't seen a user case yet.

This page certainly can be helpful so that the Upgrade pop on the MMS can redirect users here for full list of changes.