The only Chapter I am aware of that has left is Freightliner Owners Chapter (now Club). Freightliner wanted to offer the benefits to ALL Freightliner owners but the FMCA required FMCA membership, so they created their own club. I have abbreviated and oversimplified but that is the gist of what happened. I was a member and am a member of both FMCA and the FOCC. There may be others I am not a part of.
Not sure that any benefits have been dropped in recent years. Some have been renegotiated and exchanged for other providers such as road service. The Tire program alone is enough to justify my continued membership. Deep discount on six new Michelins every five years more than cover the cost of membership.