This post sounds very familiar, I will reiterate what I know. NYSP is basically abandoning the Metro21 and giving it over to the MTA PD. There may be still some special ops being used on the system, as I have seen them when they do raids in lower Westchester. There is also talk of upgrading it to P-25 digital, don't know when, but it may happen also. I installed a M-7100 base 2 years ago for the MTA PD to use for the Staten Island Terminal, and the PD is using some old MRK radios for now.
The plan is for the MTA to build the system out as they need it at all the train platforms and in between. Could be years before it is completed, could be that the 800 mhz radios they are installing now for CSP could be used on the Metro 21 when it goes P-25.
A lot of speculation, but one thing is for sure the MTA is going to use it (they are)and build it out for themselves.