I think I recall emergency services switched nationwide in/by 2010, certainly it came to the small town (i.e. far from breaking new ground) I was in around then.
As I understood it at the time, it meant everything was encrypted and you could no longer listen in. But reading your comment and some of the details there (and having studied such things since then, albeit largely forgotten) I wonder if it's similar to your description, and it mostly prevented casual listening just because it raised the bar for the equipment needed, not because it was actually encrypted.
(Afaik before that it was analogue, or at least dual or slowly switching/some services analogue.)
Listening to the scanner isn't illegal here, so long as it isn't in the commission of a crime, but the ones that are encrypted obviously would be illegal to listen into without explicit authorization.
A lot of agencies want to keep their transmissions open, but with how easy it is to turn on encryption on their existing equipment, it only takes one incident for them to suddenly decide to encrypt.
It was disappointing when I moved from an area that had all but tactical channels in the clear, to one that encrypts everything. Apparently it even caused the state agencies to say they would not respond within the jurisdiction, for a period of time, as they were locked out of being able to talk to that jurisdiction's dispatch. Ultimately they were given encrypted access.