Got a popup saying there was a threat - warning from "Apple Tech Support" real?

It’s an advertisement, and it lied, and it’a a scam.

No malware, no hack, no hacker, no virus, no malware, nothing wrong.

Ignore, learn, move on.

Read this ➡️ Recognize and avoid phishing messages, phony support calls, and other scams

iMac 2009 era has four models, with the early and mid models topping out at OS X 10.11, and later models topping out at macOS 10.13. That’s all old, or less-old.

Usual problem here can be cleared by restarting Safari with no browser tabs open (hold the shift key when launching Safari), if you’re as far back as these versions. Also consider disabling pop-ups, as ~nothing uses those now that’s not advertisements, Details: About pop-up ads and windows in Safari - Apple Support

SheilaM1960 wrote:

I received a pop up message that there is malware affecting my MacBook Pro. I have noticed an issue. It took me to an Apple Support page for a chat, but wasn't convinced it should be trusted.

:

It’s an advertisement, and it lied, and it’a a scam.

No malware, no hack, no hacker, no virus, no malware, nothing wrong.

Read this ➡️ Recognize and avoid phishing messages, phony support calls, and other scams

But…. If you entered your Apple ID credentials at that link, change your Apple ID password Right Now.

The Built in Security  is all that is required. 

I received a pop up message that there is malware affecting my MacBook Pro. I have noticed an issue. It took me to an Apple Support page for a chat, but wasn't convinced it should be trusted.

It was/is a scam, they're looking for money & passwords.