[jaas]

We (Let's Encrypt) are getting rid of subject common names and moving to just using subject alternative names.

This change has been made in short-lived (6 day) certificate profiles. It has not been made for the "classic" profile (90 day).

[tialaramex]

Have you found much trouble with clients that can't cope without CN? Is this one of those situations where anything that can't cope is also hopeless for other reasons (e.g. can't speak TLS 1.2, doesn't understand IPv6, that sort of thing) and so you can tell people you're not their biggest problem ?

[jcgl]

It’d surely be something like that. CN has been deprecated and SAN support has been required for 25 years at this point[0].

[0] https://datatracker.ietf.org/doc/html/rfc2818#section-3.1