> it could be its own, modern, safe thing

It could be a completely separate "modern, safe thing" which roughly nobody has any use for and which would be confusing since what people want are environment variables and this would instead be its own "modern, safe thing" for some reason.

Mutex<HashMap<String,String>> is probably a reasonable "modern, safe thing" for the handful of people who actually would get value out of this, and the fact it's not labelled "Environment variables" because that's not what it is probably also avoids people who needed the actual environment variables cursing because they were hidden down the back of the sofa.