People confuse themselves on this subject all the time. Expiry is optional. Is that a good idea? No. Expiry exists only to kill a certificate, intentionally,

People confuse themselves on this subject all the time. Expiry is optional. Is that a good idea? No. Expiry exists only to kill a certificate, intentionally, in a timely manner. That forces the consumer to handle their business before certificate compromise, because revocation and compromise each invoke a higher effort to mitigate to the issuer.