I will be a +1 on that request.
Just bought a .dev domain and was planning on keep it on github as a cv page, but since it needs an SSL cert (.dev rules) now I can't access it from the www subdomain. Not that I actively use it or share the url with it, but it's good to have the option. Right now the visitor will receive a cert error.
I understand not generating * certs to avoid abuse, but a www is pretty default and should be generated together with the apex domain when the repository is configured to a apex, since it's the same thing.
I think I've managed to make this work by accident.
example.com in the github pages settings.
@ on example.com domain pointing to github web servers
www CNAME @
http:\\example.com - works
https:\\example.com - works, cert good
http:\\www.example.com - works
https:\\www.example.com - cert error
For an experiment, I went into the github pages settings and set the config to be www.example.com.
My experiment didn't work, so I flipped it back.
But now https:\\www.example.com works, and https:\\example.com does too. SSLLabs is happy with both certificates (which are from Let's Encrypt).
It may be that flipping the setting to www caused a new certificate to be generated. Time will tell whether this cert gets renewed at its 3 month expiry time, or whether github forgets about it. Anyway, it seems to work for now.