1. That feature is a feature of the OpenSSL implementation, not a feature of TLS itself.

2. It's also incredibly useful for debugging SSL implementations. (Much more so than as some sort of backdoor, since that's what you seem to be implying.)

I don't know of an environment variable to enable that with OpenSSL, but there is one for NSS. For recent versions it has to be enabled at compile time, but it's enabled for the official Firefox binaries.

https://developer.mozilla.org/en-US/docs/Mozilla/Projects/NS...

Without that, you can't intercept https traffic and analyse it properly with tools like charles or wireshark. They both rely on reading the keyfile openssl et al write out when triggered to do so.

Do you have a pointer to that variable, code or doc? A web search didn't return anything obvious.