It’s so frustrating - why Chrome stops me from going to some websites.

Not all websites. Many websites are fine (like LGF.) Google’s own websites are fine.

But there are a smattering of websites I usually visit, such as Wikipedia, which now Chrome thinks is too dangerous for me to visit. The cert is rejected.

I’m wondering if some severs have been installed with software that expects something from my old iMac that it just can’t deliver?