What I have found is good is that sometimes people post old freqs on the RR forums. The posts go back at least 10 years and have helped me find things that otherwise are hard to find through the FCC ULS.
Trunked systems are archived right here on the site - go to the State page (not the County page), click on the Blue Menu button at the top right and select "Trunked Systems"
Scroll to the bottom and click on "Depreciated Systems". From there you will see a list of all of the radio systems that are no longer active in your state. You can then open the individual system just like you can from the County page.
Here, for example is the system that @Whiskey3JMC shared a link to:
Systems that are no longer active are set to deprecated status, true...however quite often they are incomplete. What I mean by that is as system are slowly being shut down or migration to a new system has begun, we often remove sites and talkgroups that are no longer valid. The end result might well result in the skeleton of what was once a much larger system in the RRDB.
The New Jersey State Police Type II system, for example, took over a year to finally shut down. In order to prevent users from downloading sites and talkgroups that were no longer active on that system, sites, frequencies, and talkgroups were removed bit by bit over that year or so. The system you see in the deprecated section now is not an accurate representation of how large the system once was.