In the two-way radio world, most protocols are open (P25, DMR, LMR, etc.) but almost every digital protocol uses the AMBE[1] voice codec, which is not.

[1] - https://en.wikipedia.org/wiki/Multi-Band_Excitation

Is it completely closed or only source-available?

It's completely closed (not even source available) but there's unlicensed software implementations available from third parties. E.g. libmbe.

AMBE in most equipment was provided in a physical chip for protection however some firmware implementations existed in hardware from big players and they were reverse-engineered.

Most of the Chinese B-brand (AliExpress stuff) radios use this reverse-engineered software implementation. They're not licensed but a Chinese company pretends to license this on behalf of DVSI though it's totally illegal. The A-brands like hytera use the official one of course.

See the popup warning on this page: https://www.dvsinc.com/soft_products/software.shtml

Since the closed nature of this codec goes counter to ham radio principles some open source codecs were also developed. The best example is codec2.

Security with obscurity mentality is rather still present at large.

We are talking about a standard over 30 years old... change happens. When it was first introduced the standard was fine the problem was never updating it. That said at least the German police uses additional crypto via smartcards.

802.11 series standards are far from open though. There are better examples, I'm sure.