Can a mid-2014 iMac run iOS 18.1.1?

You can update an iMac (21.5-inch, Mid 2014) to the latest release of Big Sur (macOS 11). You cannot update it to Monterey (macOS 12) or higher.

Big Sur is not one of the "most recent three" (Ventura, Sonoma, Sequoia) supported by vendors like Microsoft and Adobe, and does not have the latest version of Safari. However, Big Sur is enough to run the current versions of

• The Firefox and Brave Web browsers.
• The LibreOffice office suite
• The commercial Affinity V2 applications (Photo, Designer, Publisher)

iOS 18.1.1 is for iPhones, not Macs; just to avoid confusion. But macOS 11.x Big Sur is the last version your 2014 iMac can run.

Regards.

Another note on your 2014 iMac, it is considered quite old and is classed as obsolete and Vintage by Apple. What these terms mean in the Apple world is parts are largely unavailable (if repair is needed) and they will not run current versions of Mac OS.

If you are in the position, it may be time to consider its replacement. If that is an option, please let us know and we can assist with your selection. Apple has not made a 21.5" iMac in about 5 years now.

And if it’s the ever-popular and least-costly-when-sold iMac 21.5” 8 GB hard disk drive model, it’ll run exceedingly slowly on any recent macOS versions. To get it running less slowly, you’ll want an add-on external SSD, and some work to boot from the SSD. (How is included in that link.)