This means that it applies to ALL of Apple’s products assuming they continue to conform to ARM specs and that it is an existing designation rather than Apple-specific. That is to say; as Microsoft develops ARM compatible software, it wouldn’t break convention to call it “A64 Windows” when describing its compatibility with hardware.
I think you need to read:
ARM Options (Using the GNU Compiler Collection (GCC))
ARM Options (Using the GNU Compiler Collection (GCC))
gcc.gnu.org
All Arm64/A64 will give you is a rough idea of compatibility for simple binaries, and its good enough to give you a clue when the choice is "amd64" or "arm64" but for a full-blown application there are many other factors. Not just 101 different official ARM architecture versions, but all the other things that go into an Apple Silicon system-on-a-chip, like the graphics, secure enclave, neural engine etc. which could break compatibility in the future (and which Microsoft, Qualcomm, Samsung etc. may well produce their own incompatible versions of).
I think you've also got a rose-tinted view of how simple things were with Intel. "x86" has meant different things over the years, e.g. there were versions of x86 MacOS released that wouldn't run on the original "Core/Core Duo" Macs, most "x86" linux distros dropped support for anything before i686 years ago.