Comment 3 for bug 1323586

The Debian maintainer marked that as wontfix for apt, that should be handled by the client

"Unreadable sources.list files are an unsupported mode of operation
and will not be supported in neither APT nor python-apt (which
directly calls the functions from APT).

Authentication data should not be stored in sources.list files,
but in a netrc file. Using that, there's no need for unreadable
sources.list files.

If you cannot read a sources.list file, the state would be
inconsistent."