I don't think this would affect requests over an https connection.
I think you mean 10 year old software, which is a much smaller bucket than 10 year old devices. And if you're browsing the web with 10 year old software, you have much bigger problems including but not limited to committing security-suicide.
Not saying Comcast definitely doesn't use it; rather, that it'd be hilarious to see Comcast lie to everyone's faces yet again.
https://wiki.mozilla.org/CA/Included_Certificates https://docs.microsoft.com/en-us/security/trusted-root/parti...
Good luck :)
This is why TLS1.3 and HSTS exists.
That said, Comcast is big enough that it might be in cahoots with at least one less-than-scrupulous CA (or might even be a CA; I don't follow these sorts of things closely enough).
The moment that was discovered, the CA would stop being a trusted CA.
Also, given the existence of Let's Encrypt, there's much less reason to be using a paid CA, and planned migration to a new CA provider isn't too much to ask. There'd likely be some work within browsers to provide clear error messages, and sites would need some amount of time to migrate, but I think we're talking days-to-weeks before there's a warning banner on the sites and months at most before the CA is dead.
Finally, there is certificate transparency logs, you can set CT headers on your site to require the certificate to be in CT logs. Then you can monitor if anyone's creates certificates for your domain. And updated clients can validate that certificates is in the CT logs.
HTTPS is pretty robust these days. There's still a few corner cases around SNI, but that only leaks what host your visiting, wouldn't allow injection -- and specs are slowly closing those holes too.
But really, if you don't trust what installed on your machine it's game over.