Is my 2017 iMac still protected under Monterrey MacOS 12.6.7?

If the alert is appearing in the upper right corner of your Mac's display, then it is a completely fraudulent, spurious scam. To stop those Notifications from appearing read Stop unwanted Notifications - Apple Community.

That User Tip also describes the reason for their appearance. It's not the work of a hacker. Unless you are that hacker 😄

On the other hand if that User Tip does not describe what you are encountering, then I will need more information. Needless to say "Norton" is junk that should never be installed on any Mac, but if you in fact installed it you may need some help getting rid of it.

1. No, the best macOS is always the latest.
2. Apple continues to support Monterey.

I don't know what you mean by "protected" though. Protected from what?

During the last 10 days I have been receiving alerts to load Norton Antivirus to protect my iMac, which by the way, I cannot shut down the pop ups that come out every 10 seconds (or close to that frequency).

To stop unwanted Notifications please read Stop unwanted Notifications - Apple Community

If your Mac is performing poorly something else is going on. Please determine if the same problems occur in "Safe Mode": Use safe mode on your Mac - Apple Support.

Follow the previous excellent advise - First

If Apple continues its' past practices, macOS 12 Monterey will continue to receive Security Update for 3 Years from Date of Release

In that case, macOS 12 Monterey was released October 25, 2021.

Therefore, the Security Updates should continue until some time in 2024

Prime example >>About the security content of macOS Monterey 12.7

Keep your Mac up to date

Additional Links pertaining to Security Built into the Operating System

Mac app security enhancements

macOS - Security - Apple

Depends how you define "best". The user interface with Ventura and probably Sonoma has changed for better or worse but Monterey is still supported with all security/bug updates. So it is your choice which to use for now.

Julio M wrote:

1) Is Monterrey still the best system for my iMac?

Is there something in Ventura that you desperately want to have? If not, then Monterey will be fine.

2) is my iMac still protected.

Protected from what?

I assume you are asking about malware. That's a myth. Don't believe what you see on the internet. Mac's can't "get" viruses or other malware. Users must bypass multiple levels of Apple security protections to install malware. But it is the user that must install the malware - on purpose. Usually the hope is to run some expensive software for free or watch some movie for free. You won't get anything for free that way, but you would get malware. So if you don't do that, you'll be fine.

You may have visited a website that you allowed to push notifications and are getting the spam from Norton. To remove it go into Safari/ Website/Notifications settings pane and and do the following: