Apple says “not all known security issues” are patched in updates for older OSes
arstechnica.com
arstechnica.com
I don’t think Windows 95 gets all necessary security updates, for example. Nor gets Linux 1.0 (you could backport them yourself, of course)
What makes this much more an issue with Apple’s OSes is that they obsolete hard- and software versions way more aggressively than others. Older hardware won’t support newer OS versions, and that, if you can move to a newer OS version, upgrading will more often break your existing programs.
This article talks about older OS versions that are listed as still receiving security updates, but that that those updates aren't actually patching holes in all supported versions equally. It's like Microsoft released security patches for Windows 11 and 10 while both are in support, but without telling anyone actually omitted some fixes from the patches released for Windows 10.
While they often support devices (particularly iOS devices) for a long time, I always seem to get the iPhone or Mac that gets the unusually low amount of support.