That would mean installing packages that are not bearing valid signature. If you don’t want package signing, simply disable signature checking altogether in your pacman configuration. Accepting all the security implications, of course.Pacman could do with a feature to bypass authors packages and keys so those don't disrupt updates.
An update being impossible due to invalid signatures is an intended feature, not a bug.
Attachment:
OpenPGP_signature
Description: OpenPGP digital signature
