I disagree that TLS is a good example of a breaking change, since this is a specific library dealing with a developing and dynamic area of security. If this weren’t changed, it would compromise PKI cert integrity by allowing obsolete fields. Also you can get the old behavior back with a flag. The fact that other languages are lagging here reflects badly on them.

As discussed in the link, CommonName has been deprecated in x.509 serverAuth certificates for decades, and all major browsers dropped support for the field (even as a fallback) years ago.