Comment 19 for bug 231316

Emilio, I've just used the following logic: if "bts" is not configured (which it is now by default), or is "ubuntu", the error with the link to ubuntu-bug will appear.

That keeps existing "bts=debian" installations just working and in case of new installs, "bts" is undefined now: you have to configure it to "debian" to make it work. IMHO that's as good as some hidden ACK-style file.
The only issue might be with upgrades, where bts is "debian" already (e.g. in /etc/reportbugrc (and the admin does not use the new conffile) or ~/.reportbugrc), but the user still thinks it would send the bug to Ubuntu.

The package/mechanism can still be improved, of course. But for now I think it's the best solution.