The info page for shred includes this example code:

     i=$(mktemp)
     exec 3<>"$i"
     rm -- "$i"
     echo "Hello, world" >&3
     shred - >&3
     exec 3>-

The last line is incorrect.  It should be "exec 3>&-" which closes file
descriptor 3.  As written, the example code creates or truncates a
file literally named "-" instead.

This same error appears on
https://www.gnu.org/software/coreutils/manual/html_node/shred-invocation.html
(as of today) so I believe it's an upstream bug.

Fixed upstream.
https://github.com/coreutils/coreutils/commit/d39c52372

Thank you.
Pádraig.