I believe I made the suggestion in another thread to create a new database field for date of last reported activity on a frequency or Talkgroup. Apparently the higher ups felt that would be too complicated and may not work very well for various reasons.
Like other commenters said, it is a complicated issue because many agencies have frequencies or talkgroups programmed that are rarely used. It's basically trying to prove a negative.
I will guarantee you, a lot of of public safety users are not aware of what half the channels in their own radios are used for, and they don't want to know.