Am I still safe on my late 2015 i mac running Monterey?

If you are running macOS 12.7.6 you are on a pretty safe OS.

If you are running 12.2 you are not.

Here are a few examples of the number of vulnerabilities within some recent versions of macOS:

macOS version           vulnerabilities
----------------------------------------
12.2                    1041
12.7.6                   543
13.2                     703
13.3.3                   612
14.0                     457
----------------------------------------

( Source CVE analysis ).

As you can see, this number doesn't regularly goes either downward nor upward.

From a long experience on OS vulnerabilities, I would say that on the whole, since the volume of code is increasing,

the bugs number does mathematically increase proportionally ( this is basic probability ).

On the other hand, some OS editors are working hard to increase the real security of the OS basics ( the filesystem, the memory protection, the inter-processes protection…).

Some others just make a lot of talk on the hot subject of cyber security.

Next, you have to plan the replacement of your iMac within the few next years. 10 years old is too much for the

actual quality level of hardware components, and most notably the one subjects to physical fatigue.

Here is my honest opinion to this question. You'll most likely be ok. I know many users still running much older systems with few issues. With that said, anytime an OS looses support, there is an inherent risk to using it. It no longer receives security updates to address known vulnerabilities. The longer an OS goes without support, the more vulnerabilities are discovered. As such, as more time goes on, the riskier it is to use it.

I'm going to recommend that you update to a newer computer with a supported OS at some point. It's always better to not chance it when it comes to your personal data in my opinion. Especially if you have sensitive information on this computer. If you really can't at this time, just take precautions while using the internet.

Best,

nah, just because apple stop supporting the operating system, does not mean it turns on some beacon of light altering all would-be hackers and your computer is now ripe for the plucking. to have people exploit any variabilities it might have would require you to put yourself in a position for those to be accessed