An issue in the css-env spec says:

ISSUE 3. Define the full set of places env() can be used.

• Should be able to replace any subset of MQ syntax, for example.

This made me think if we could define custom environment variables and use them in @media queries, would there still be a need for @custom-media or would it become redundant.

That could look something like the following:

@env --narrow-window: (max-width: 30em);

@media env(--narrow-window) {
  /* ... */
}

(The syntax could be different, of course, as the syntax for custom environment variables hasn’t yet been decided.)