Apple is asking you to throw away your devices, not Zig. Getting proper CI coverage for supported versions of the OS is already pretty expensive and doing the same for unsupported systems is entirely unfeasible at this moment.
Don't buy Apple if you don't want to throw away functioning hardware.
Having 'we support proprietary systems until their makers support it' as a hard rule is unnecessary and harsh. It is a simple rule, but I don't think it is good. Why not choose the supported systems on individual merits, e.g. on usage statistics? I think for example the Linux kernel does that.
edit: I just continued the previous thought. If your reply means that you are open or likely to target deprecated systems in the future, that's much better!
It would be nice to eventually provide support older systems but first we need to get to a point where doing so doesn't mean taking away resources from more worthy endeavors.
Not supporting proprietary OSes is a bummer. Especially if it is marked as deprecated since it is unlikely to change. People choosing Zig to anything choose their users to throw away their devices.
.. the number of hardware and software just keeps growing and growing endlessly, and the number of their combinations grows even faster. We probably need some more virtual machines as targets to cover them all.