Software Updates and Software Upgrades ⇥ support.apple.com
Updates are more frequently released software patches that help secure or enhance the current operating system and are designed to protect your device against security or privacy vulnerabilities. Updates use a second and sometimes a third number to denote increments. For example, iOS 15.1 is an update to iOS 15.
For upgrades of iOS, iPadOS, tvOS, and watchOS, a simple integer is used — for example, iOS 15 and iPadOS 16.1. […]
It is not a great sign when official documentation misuses the word “integer” in a sentence that immediately negates the previous paragraph’s explanation for how Apple differentiates software updates and software upgrades.
Apple has finally officially state their stance on the latest operating system is the only fully patched one. If you are running anything other than $latest you are likely vulnerable.
This has been indicated, though never confirmed, for years; it is now documented. Earlier this year, Apple gave iOS 14 about four months of extra support. Vulnerabilities in previous versions of MacOS persist for months after being patched in newer versions, if they are fixed at all.