I can't find anything recent that would explain AA's removing this nicety.
Mrs alien & I obtain status through liesure travel mostly in coach. This past week we asked an agent about this @ checkin. She said she was able to block the seat between us only against kiosk checkin.
I believe this ability to block adjacent seating of an elite until it is the last available seat is a huge perk. I assume it cost AA nothing and is a simple software issue. Why would AA do away with it?