All of a sudden? I remember some agencies in Indiana using voice inversion scrambling on VHF back in the 1980's. I suspect that it was a wide-spread though not commonly used practice. I assume they were doing it for "officer safety" because even simple receivers and scanners could monitor their comms.It was never an officer safety issue for decades, now all of the sudden it is?
Law enforcement agencies have been trying to keep the public from listening to their comms for years. With modern systems being sold with encryption capability, it's just easier to implement now.