Is Monterey 12.7.6 safe?

Monterey is the last possible macOS for a late 2015 iMac. See the following from Identify your iMac model - Apple Support

iMac (Retina 4K, 21.5-inch, Late 2015)

Model Identifier: iMac16,2

Part Numbers: MK452xx/A

Newest compatible operating system: macOS Monterey

and

iMac (21.5-inch, Late 2015)

Model Identifier: iMac16,1

Part Numbers: MK142xx/A, MK442xx/A

Newest compatible operating system: macOS Monterey

macOS 12.7.6 (including Safari 17.6) was released July 29, 2024. See: Apple security releases - Apple Support

Apple has been releasing security updates for the current macOS and for two previous versions so it is likely that there won't be future security releases for Monterey after Sequoia is released later this Fall. You will however be able to run current versions of 3rd party internet browsers after you can no longer get a current Safari browser.

Apple's own policy is here: About software updates for Apple devices - Apple Support (CA)

Basically, the current version gets all the support updates and security patches. The previous two versions get support updates (not necessarily all) but probably all critical security patches. Older versions then that may still get security patches for some period - what exactly will vary (but for one example, even El Capitan still gets XProtect database updates).

cambria31 wrote:

I have a 2015 late model iMac. I believe it's no longer supported. Will Monterey receive any security updates?

Any? Yes. Apple has continued to provide XProtect updates for operating systems as old as Yosemite, at least. The last update was issued a couple days ago. Yosemite is seven macOS versions prior to Monterey and those Macs are over fifteen years old.

Should I be concerned or is it safe under all or certain circumstances?

No. Nothing is safe under all circumstances. Life is not safe. It's not safe to get out of bed. Staying in bed is even less safe, depending on who else is in it. Countless threats to your safety have always existed, and they will always exist. However, you can mitigate those threats to an acceptable level, no matter what computer you have or whatever operating system it uses.

Should you be concerned? No more than you have ever been concerned. You can be proactive though, and remain vigilant. Read Effective defenses against malware and other threats - Apple Community.

cambria31 wrote:

I have a 2015 late model iMac. I believe it's no longer supported. Will Monterey receive any security updates? Should I be concerned or is it safe under all or certain circumstances? Any insight or advice appreciated!

What do you mean by "safe"?

Do you mean that one day it will be taken over by hackers, with no action on your part? Impossible.

Or do you mean that if you try really, really hard to purposefully install the latest malware that you can find on the dark, nether regions of internet, it will be slightly easier? Then, yeah. That's possible.