Is my late 2015 iMac still safe to use for online banking and shopping?

MacOS 12 Monterey, the last OS that 2015 iMac can run, is three generations behind the current OS 26 Tahoe and its support from Apple is waning. It was last updated 16 months ago.

Online security of your iMac is still fine since that security gets baked in to the browser, and Safari is still secure. When Apple finally stops supporting Safari on Monterey, if it hasn't already, then you can still turn to other supported and secure browsers such as Brave or Firefox for a time.

The Mac mini can be a powerhouse if properly configured at purchase.

Alas, you cannot use the beautiful iMac screen as a monitor for the mini, or any other computer. But a good quality monitor can be purchased for the mini at reasonable cost. And when the mini has served its time, you can continue on to your next Mac and take the separate monitor with you if you like.

Target Display Mode went away when the first 27" 5K Retina iMac came out in Late 2014. It never returned.

There is a workaround product, called Luna Display, by Astropad, that might let you use that 27" 5K iMac screen as a second screen for a new Mac. You would still need at least one real hardware display. The Astropad people themselves acknowledge that Luna Display has limitations – even if you run a USB cable between the Macs to help carry video traffic, Luna Display would not be suitable for high motion applications like video editing or gaming. There also might be lags and pixelation when things change on the screen.

So it would be best to get a real display for a new Mini with the expectation that this would be your only display. If it turned out that you could live with Luna Display's limitations and also use the iMac as a second display, that would be gravy.

The last major version of Safari that Monterey was eligible to run was Safari 17. The most recent version of Safari 17 was Safari 17.6, released on July 29, 2024 – unless there was some patch that wasn't mentioned on the main Safari Release Notes page.

Safari Release Notes | Apple Developer Documentation

Monterey itself seems to have gotten a security update (Monterey 12.7.6) on July 29, 2024.

Apple security releases - Apple Support

Then there are XProtect updates. If you run System Information, then click on the Installations sidebar item in the Software section, you'll get a list of applications. Sort by Software Name and look for XProtectPlistConfigData. On my Mac, there are a ton of entries with that name; the most recent has version 5323 and was installed on November 11, 2025. I don't know if Apple still distributes XProtect updates to Monterey systems, but if they are doing that, you should also see an entry something like that.