Why it matters: When Apple launched iOS 15 last fall, it promised that users who didn't want to upgrade from iOS 14 would still get critical security updates. That option seems to have recently disappeared, as Apple confirmed this week it always intended to get all iOS 14 users onto iOS 15.
The last security update for iOS 14---iOS 14.8.1 (along with iPadOS 14.8.1)---rolled out in October, but this week 9to5Mac reported that iOS 14 users could no longer update to that version. The only option that appears now is 15.2.1. Apple then confirmed to Ars Technica that this is intentional, and that the ongoing security updates for iOS 14 were only meant as a temporary grace period.
Apple never mentioned this when it first announced and launched iOS 15. In fact, the features list for iOS 15 as of this writing still says (near the bottom) iOS 14 users can still get security updates without mentioning a limited period.
Conversely, Apple offers the latest macOS security updates for the most recent version of the OS---Monterey---along with the previous two, Big Sur and Catalina. Catalina is a year older than iOS 14.
The only difference with these last two iOS versions is that iOS 15 is compatible with every iOS 14 device, so there are no hardware conflicts. Still, Apple could've been a bit less opaque on its security update policy.