When is macOS Big Sur EOL, meaning there will be no more security fixes?

I have a relatively new MacBook Air (13-inch, Early 2014), which works wonderfully and is in daily use.

I was shocked to discover that macOS Monterey is not supported on my relatively new machine (macOS Monterey is compatible with these computers). Are there any technical reasons for this? Or is the only reason to force the sale of new devices?

When is macOS Big Sur EOL, meaning there will be no more security fixes? I did not find a lifecycle document with specific dates at Apple KB.

Solution 1:

Apple does not provide that kind of information. However, the materials needed to make a guess can be found on this site (source, it's MIT licensed). From the information as far as I can tell on this site, it seems that support will be provided for basically three years from the release of it's version.

This is just my opinion, but I wouldn't call a 7-year-old PC a "relatively new". It's old.

Solution 2:

There do not appear to be technical reasons for not allowing you to install macOS Monterey on your MacBook Air (13-inch, Early 2014), as you can install macOS Monterey on your device in an unofficial way. Performance may not be great, but (in my opinion) that should not block you from installing it (though a warning message would be understandable). There were some devices that Apple dropped that did have issues with the new update, but the OpenCore Legacy Patcher developers were able to fix them, but your device was never one of the models affected from what I can tell.

Nobody knows for sure when macOS Big Sur will stop getting security updates, but as Skye-AT and Tetsujin mentioned, Apple usually provides security updates for OSes up to 3 versions prior, so you probably still have at least 3 years of support.

When that ends, if you wish to install macOS Monterey, OpenCore Legacy Patcher supports your MacBook Air and macOS Monterey, so you can follow their instructions. Warning: I have not attempted these steps (fortunately my Mac is from 2015), so do so at your own risk (and feel free to use any up-to-date guide at that time.) The final version of macOS Monterey released just a couple days ago, so there still may be some undiscovered bugs running it on unofficial hardware, but it appears so far there are no bugs on your model.

Solution 3:

The "rule of thumb" is 10 years - 5 years full support followed by 5 years partial support.

Apple publishes a list which covers hardware repair. Software support generally but not absolutely follows this list.
Getting a service for your Apple product after its warranty has expired

In general, if it's on the Vintage list it will still get security updates even after it no longer gets full OS upgrades. Once on the Obsolete list, support is EoL.

Historically, Apple have issued regular security updates for OSes up to "three prior". Currently, Mojave still gets security updates, earlier OSes don't. [That is likely to change as of now, as Mojave becomes "four prior".]