All the places in the standard library that explicitly write output or error messages to sys.stdout or sys.stderr. (There are far fewer places that explicitly take input from sys.stdin, so there's that, I suppose.)

> it would be far more confusing when stdin/stdout/stderr were sometimes text sometimes binary

I am not suggesting that. They should always be binary, i.e., streams of bytes. That's the lowest common denominator for all uses cases, so that's what a language runtime and a library should be doing.

> If you meant that they should always be binary that's also unoptimal. In most use cases an user works with text.

Users who work with text can easily wrap binary streams in a TextIOWrapper (or an appropriate alternative) if the basic streams are always binary.

Users who work with binary but can't control library code that insists on treating things as text are SOL if the basic streams are text, with buffer attributes that let user code use the binary version but only in code the user explicitly controls.