[kazinator]

Sourced environment scripts in the Unix environment are standard operating procedure. E.g. for toolchains.

The .env being evaluated as a shell script means that it's in a widely used language, with a widely known syntax. You can look at it and know what it's going to do.

The .env being a data format to some uncommon utility; that's anyone's guess.

For instance, suppose we want a newline character in an environment variable. Does the given "env file" format support that? How?

There is one de-facto standard format: the /proc/<pid>/environ kernel output format on Linux. The variables are null-terminated strings, so it is effectively a binary format. It represents any variable value without requiring quoting mechanisms.