> Is it as simple as a header with the version number, or does `erc-version'
> need to respond with 5.3?

Yes, a header with the version number is enough. `erc-version' is
irrelevant to package.el.

> > We need to either ask these packages to rely on Emacs version instead
> > of erc version, or come up with a way to indicate that a package
> > should inherit emacs' version number, or go back to specifying erc's
> > version.
>
> I think we should definitely ask these packages to not rely on
erc-version 5.3, but we can also try and keep the obsolete version number
around until it doesn't break other packages.

Ok, let's bring back the number for now. I'll try to get in touch with the
authors.

I think it would be a useful feature for built-in packages to be able to
specify "follow emacs version". This could be as simple as specifying
"emacs" on the Version header (instead of a version number).