Yosemite End of Life

Apple does not reveal this information, but it's probably a good guess that OS X 10.10 Yosemite will not be supported for long. You should upgrade, if you can. Upgrades and updates contain important security improvements plus additional features.

Apple for the last several years, has maintained a three-year sliding window of security, and selected application updates to the current, and the last versions, of the preceding two annual operating systems. Unless this pattern changes, and with the release of High Sierra, I would expect to see the new support window as: 10.13.n, 10.12.6, and 10.11.6. I show the last released version for Sierra, and El Capitan because this is usually the criterion for receiving any notifications of updates for them.

Apple does not publicly state that an operating system is end-of-life, or unsupported — it just stopped providing updates for it.

So we can assume Apple will not be issuing a security update for Yosemite regarding the Meltdown and Spectre vulnerabilities.

What is more of a concern is when El Capitan becomes unsupported next year working on your premise of supported releases I will be entirely open to threats on my Early 2008 24" iMac which officially does not support macOS Sierra.

Apple alone makes a determination of their retrospect product support. My “premise” is based on several years of Apple support pattern. You can assume that Apple is about change, not position. This creates islands of unsupported products at some point.

With a ten-year old iMac, it may abruptly retire itself before you are ready to make that decision, and operating system support will become a moot point.

It may well be a ten-year old iMac but it is still functioning as well as the day it was new and I paid a substantial amount when it was purchased.

The 'island of support' for Macs is nothing new. I know people with Intel Core Duo machines that can not run any release newer than OS X 10.6.8 Snow Leopard and they are left in the cold as Apple pulled support for Snow Leopard some time back but the machines are otherwise sound and remain productive twelve years on.