Certificates are not trusted on Chrome and Safari on old iMac with El Capitan

[jsha]

@DrJa

But what of the many friends and customers who use Chrome on MACs they purchased 5 years ago?

OS X Yosemite (10.10) and El Capitan (10.11) didn't trust ISRG Root X1 and so won't validate Let's Encrypt certificates. Sierra (10.12), released in 2016, does. According to Wikipedia, Sierra can run on:

• iMac: Late 2009 or newer
• MacBook and MacBook 12-inch: Late 2009 or newer
• MacBook Pro: Mid 2010 or newer
• MacBook Air: Late 2010 or newer
• Mac Mini: Mid 2010 or newer
• Mac Pro: Mid 2010 or newer
• Xserve is no longer compatible.

@DrJa

Yosemite won't update. Chrome won't update.

I'm not sure why your friends and customers aren't able to update Yosemite or Chrome, but it would be good to contact Apple support about the problem. In particular, people who aren't receiving updates to their browser are at risk for malware, since each new browser release usually fixes some serious security bugs.