I just ran into this issue on Linux when piping a binary file to stdin resulted in a UnicodeDecodeError while trying to read a byte from the stream. Passing /dev/stdin is a workaround that does not require modifications to an application.

As for the proposed PR 13165, I'd suggest to gracefully fallback to normal stdout/stdin if the buffer is not available. That approach is also followed in the fileinput module, and takes care of the note for library developers in the documentation at https://docs.python.org/3/library/sys.html#sys.stdin

Not feeling particular strong about the graceful handling, but I hope that the test code can be simplified in that case.