RadioReference has discussions in depth.

There is a modulation scheme called ProVoice that's not decodable by hobbyist scanners.

And sometimes some agencies encrypt everything.

In Colorado, so far, only some tactical channels are routinely encrypted. And ProVoice hasn't worked out well so it's been turned off where tried here.

But it's always a risk.
OTOH it's also a risk that the encryption on the system won't work for the users either.

I use my scanners twice daily for the commute so it's well worth the $$ to stay on top of things.

Heard the big grassfires near Boulder paged out last week (for instance)