It seems that Let’s Encrypt does not recommend revoking an old certificate after it has been renewed. I wonder why, since it seems that revoking it could have a number of benefits:
You could supply a contact email for use in special circumstances without having to deal with expiration email for a renewed certificate whenever the set of domains in your certificate changes, or for no longer needed certificates. (I don’t know if Let’s Encrypt sends expiration email reminders for revoked certificates.)
The CA could more directly monitor certificate renewal status if the CA needs to initiate revocation, like in the recent incident.
It could potentially reduce load at the CA for generating OCSP responses, because a revoked certificate only needs a single signed revoked response, whereas a valid certificate needs a continuous stream of signed good OCSP responses.
So what are the downsides? Is it complex to implement correctly in ACME clients? I can imagine you need some extra verification that your new certificate is fully deployed before you revoke your old certificate.