The NCTCOG sites in Collin, Denton and Dallas counties are, indeed, still active. Some time back, the Tarrant County site was moved to the Dallas side as site 23 - directly connected rather than over ISSI. I'm not sure about the status of the Tarrant site as I'm too far away but I assume it remains active, as well. The P25 controller for the DFW Airport NCTCOG site was repurposed and is the current controller for the DFW Airport P25 system.
The Dallas P25 system and NCTCOG share the same controller and Sys ID. The RR database reflects that (the Sys ID is the database key so we don't want/can't separate them). However, in my scanners I have Dallas P25 and NCTCOG is separate favorites lists and it works quite well. The talk groups don't cross between Dallas P25 and NCTCOG systems (a sysadmin function).
I've run UniTrunker on both these systems and there is encrypted traffic on the NCTCOG system. I suspect that it is Fed traffic and may explain why we hear so little FBI, DEA, US Marshall, etc. traffic on the local Fed systems compared to what we used to hear.
Russell