It’s a basic optimisation. Java does it as well, for example, and it’s safe. But it’s not part of the type system and it’s not what people here are talking about.

It's more complex in practice, since you need to also check that the compiler needs to see the entire lifetime of the variable to check if it's not modified asynchronously, as you say. But for many common use cases, this is not hard to confirm, especially since the Go compiler has access to the entire source code of the entire program (unlike in C++, Java, etc).