That's fine; Apple's under no obligation to have perfect security updates for older versions, and I believe many Linux distros, including the main ones (Ubuntu and Debian comes to mind, though from a while ago) have similar issues with poor backporting of security patches.
I just think more tech people should be aware of this. Now, the "responsible" thing, apparently, is to wait a week or two to see if a major update is bricking devices, and install it if not; and to install minor updates immediately. That would come as a surprise to many people who see themselves as Mac experts, who are more likely to delay updates. Tons of companies also delay updates for many months.
Apple should communicate more on this; and they should consider changing their update schedule. Windows doesn't release a new version every year; you'll run into this issue less than twice a decade, compared to yearly for the Mac.
Windows does nowadays (since Windows 10)... see the major (now yearly) updates.
The major Windows releases since the Win10 OG release: 1511, 1607 (available as LTSB for businesses, Server 2016), 1703, 1709 (first ARM64 release, for the desktop SKU), 1803, 1809 (available as LTSB for businesses, Server 2019), 1903, 2004, Iron (Server 2022, not shipped on desktop), 21H2 (Cobalt, corresponding to Windows 11 on desktop).
One aspect is that an older OS might have the headline vulnerabilities patched, but not architectural bugs or additional security hardening. Those aren't generally backported to older releases.
With some other companies (typically more business oriented), you can know the moment you buy their software how long it will be supported. See for example https://docs.microsoft.com/en-us/lifecycle/faq/windows.
For Apple, there’s https://support.apple.com/en-us/HT201624, but it only lists how long hardware is supported (that list does go back a long time, though. It’s good to have official confirmation that the Macintosh 128K is obsolete)
For Mac OS, the best you get are educated guesses from third parties. For example, https://eclecticlight.co/2021/09/22/how-long-does-apple-supp... says
“As far as macOS goes, everyone will tell you that Apple supports the current version for about a year before it’s replaced by a new major release, then provides two years of security updates for it. The strange thing about that is Apple doesn’t seem to have committed that to writing, and I’ve searched long and hard for its official policy on many occasions”