They're only not supporting new features which have specific requirements of the device — the features it had when you bought it are still supported. The listed hardware support goes back to 2015 and historically they've continued shipping security and stability updates for the previous OS release, so whichever of those old devices are still running won't leave security support until the release of whatever comes _after_ Monterrey (presumably in the fall of 2022) when Big Sur falls out of support.
They're only not supporting new features which have specific requirements of the device — the features it had when you bought it are still supported. The listed hardware support goes back to 2015 and historically they've continued shipping security and stability updates for the previous OS release, so whichever of those old devices are still running won't leave security support until the release of whatever comes _after_ Monterrey (presumably in the fall of 2022) when Big Sur falls out of support.