I had suggested to the IREACH Board a year or so ago when they were considering a patch petition I presented that they adopt a tone code for IREACH as part of the narrowbanding. At the time there was discussion of 156.7 but I think there were users in nearby states already using that on 155.055 or one of the adjacent channels that would have been an problem.
The DPL code was chosen since it is more unlikely that others would be using it on the actual or adjacent channels. Also, since IREACH is solely an Illinois operation the use of 156.7 is not needed.
As with the adoption of new tone/codes for mutual aid and shared channels (IFERN, Firegrounds and others) it is standard procedure to set it up as PL/DPL transmit only until a certain date when all transmitters are supposed to be sending the tone then users can start adding it to the receive side of the radios. This way users don't get locked out of messages they may miss due to the transmitter not having the proper tone.