I doubt it's actually possible to enforce encryption backdoors beyond a few major vendors. The result would be similar to exiting attempts to prohibit reverse engineering. It's impossible to outlaw debuggers, disassemblers, logic analyzers, and similar tools. It's like outlawing radios that can tune in to any station. It's been done, but it's not all that effective.
Even if all software from major vendors like Microsoft, Apple, and Google implemented protocols with backdoors, correct implementations of the underlying algorithms are necessary for those to function.
We've seen forced decryption laws in the UK. Forward secrecy basically defeats RIPA, because you can't force someone to decrypt something they never had the key for in the first place.
China has attempted to regulate cryptography, essentially requiring a license to develop, buy, sell, or research encryption. They have mandatory key escrow too. It's useless. Everyone uses encryption all the time. There's no putting the genie back in the bottle.