The message sorting is not RCS. It is another Google Messages feature which can be disabled in Settings->Message organization. It has been available for quite a

The message sorting is not RCS. It is another Google Messages feature which can be disabled in Settings->Message organization. It has been available for quite awhile but not the default. Possibly they decided to make both features the default at the same time. As a side note, it actually does a pretty good job of categorizing my messages.