Modern browsers are going out of their way to hide every bit of information about the website (including even the URL) — so I don't know how these customers would actually even find out what CA issued the certificate.

In Safari, I don't even know how to find that information anymore. When I want to check expiration dates for my own sites, I start Firefox.

It’s the symbol to the left of the URL > Show Certificate. They even make it available on iOS Safari (Page Info > Connection Security Details), but if it’s expired, you’ll know by the big red warning page.