It's a trivial function: https://cs.opensource.google/go/go/+/refs/tags/go1.23.1:src/...

You could write your own errorsJoin() and change Error() method to suit your needs.

But really in this particular scenario you would be better served by something like:

    func errorsConcat(err1 error, err2 error) error {
      if (err1) {
        return err1;
      }
      return err2;
    }

And then do: err = errorsConcat(err, f.Close())

In the scenario described in this article, errors.Join() would most often reduce to that (in terms of what Error() string would produce).