can an iMac 27" (4K) late 2015 get security updates?

macOS Monterey 12 is no longer supported, however, there's no reason not to use it with normal care.

Effective defenses against malware and ot… - Apple Community

Recognize and avoid social engineering schemes including phishing messages, phony support calls, and other scams - Apple Support

I still have a 27" Late 2015 running Monterey I still use on occasion.

jinet wrote:

But, does your machine still permit security updates, not software updates? I guess the answer would be, No. No longer supported means no security updates.

This is a user-to-user forum. You are not speaking to Apple ("your machine") here.

Monterey no longer gets the latest versions of Safari. Version 17 is the last major version that is compatible with Monterey, and as of today, Version 17.6 (released July 29, 2024) is the most recent release of Version 17. There was also a Monterey release (12.7.6) with a number of security fixes on the same date.

Apple security releases - Apple Support

Safari Release Notes | Apple Developer Documentation

You could install a third-party browser. The current version of Firefox will run on Catalina and higher, so it seems likely that Monterey will be eligible to run new releases of Firefox for at least a couple of years.

Mozilla – Firefox

Also keep in mind that a lot of today's criminals find it easier to target the "weak link" behind the keyboard (i. e., the user!) than to target security flaws in the computer itself. Even running the latest version of macOS will not protect you against these "social engineering" and "phishing" attacks, because if you fall for these attacks, you will be compromising your own security. The only good defense here is awareness and education.

Recognize and avoid social engineering schemes including phishing messages, phony support calls, and other scams - Apple Support

jinet wrote:

Thanks. I will pass the information as a manual on to my mother along with the machine.

You're welcome

But, does your machine still permit security updates, not software updates? I guess the answer would be, No. No longer supported means no security updates.

Correct. I wouldn't worry about it. Unless your mom is the head of a multinational corporation or is somehow a target of online evildoers.

A hail Mary: there is no way to use the screen of the iMac as an external display, correct? Such a shame; it's a nice big display.

Also correct.

jinet wrote:

A hail Mary: there is no way to use the screen of the iMac as an external display, correct? Such a shame; it's a nice big display.

Although there is a workaround (with its own compromises), there's no way to use that 27" 5K Retina iMac as a first-class substitute for a hardware monitor like the 27" 5K Apple Studio Display. The old Target Display Mode went away when the first 27" 5K Retina iMac came out in Late 2014, and never came back.

For a long time, there were only two standalone 27" 5K displays – the $1600+ Apple 5K Studio Display, and a LG display that often went for over $1000. Now we are starting to see 27" 5K (5120x2880) displays from a few more vendors, including Samsung, BenQ, Asus, and ViewSonic. I believe that some of these displays might have street prices as low as $800 – $950.

27" 4K displays continue to be a lot cheaper – you can get some that have IPS panels and near-100% coverage of sRGB for as little as $300 – $350. They're not as sharp as the Apple 27" 5K Studio Display or your 27" 5K Retina iMac screen, but given the difference in price (and selection), a lot of people have wound up settling for them.