This is about removing the three pseudo commands $cmd.sys.inprog, $cmd.sys.killop, and $cmd.sys.unlock. As of SERVER-7775, they have been available as real commands currentOp, killOp, and fsyncUnlock. All of the code for the $cmd.sys.* variants was marked with "TODO remove after 3.2" but that hasn't happened yet.

Just to be clear, this has nothing to do with the protocol for sending normal commands by issuing a find on db.$cmd. That will continue to work fine. This is just about the 3 special cases that used to use $cmd.sys.nameOfCommand.