Here is a tested illustration of how I would suggest to manually handle encoding with check_output():

>>> text_input = "©"
>>> args = ("iconv", "--from-code", "ISO_8859-1", "--to-code", "UTF-8")
>>> bytes_output = check_output(args, input=text_input.encode("iso-8859-1"))
>>> bytes_output
b'\xc2\xa9'
>>> bytes_output.decode("utf-8")
'©'

If you wanted actual universal newline translation (which is more than plain text encoding), it would be more complicated, but still possible.

But I am not hugely against adding new “encoding” and “errors” parameters. The reasons against are that I don’t see it as necessary, and there are already a lot of subprocess-specific options to wade through.