In addition, people looking at this is exposing a bug in many clients, where even if there is a name that would fit in the CN, the client is putting the first supplied name in the CN regardless of length. But this would have not worked with prior Let's Encrypt behavior either.
Sure, but I think that clients should consider providing an option to specify a CN. There was the case earlier this year where Let's Encrypt's server changed the behavior of which name was put in the certificate CN if the CSR didn't include one, and that broke some people's workflows because they didn't realize that their client wasn't setting it in the CSR.