That sounds impossible to enforce. Key signing is done offline. If I have an expired signing key, what’s stopping me setting my clock back a few years, creating a new signature and signing it?
If the resulting key works indefinitely, the expiration date on my signing key is utterly meaningless.
Microsoft's Authenticode has been doing this for a long time, allowing a signature to be considered as valid long after the signing certificate expired.
And what stops people from just logging into the bios and changing the date or pulling the clock battery? The point we're all making here is, at least for this application, is that an expiration is pointless.
If the resulting key works indefinitely, the expiration date on my signing key is utterly meaningless.