Configuraton:

• OS: Arch Linux
• pycodestyle version: 2.6.0
• Python version: 3.8.3
• Installation source: Arch Linux packages

W504 (line break after binary operator) shows up when the slash that indicates positional-only arguments (PEP 570) is followed by a newline. For example:

def really_long_name_for_a_function(
    very_long_argument_1, very_long_argument_2, /
):
    pass

pycodestyle thinks that the slash is a binary operator. I can, however, silence it using a comma after the slash, as follows:

def really_long_name_for_a_function(
    very_long_argument_1, very_long_argument_2, /,
):
    pass

I think this is an issue, as the slash isn't a binary operator, and the comma seems like a workaround for arguments on a single line than good practice (at least in this case).