No one outside of Apple knows what plans they have - if any - to add raw support for any particular camera. It is worth noting that it does not appear to be a particular priority these days, going on past behaviour.
such a simple fix as the z9 is exactly the same operating system
It is not a "simple fix" and the operating system of the camera is of zero relevance. The raw support for each model has to be written independently, that the Z9 is partially supported is of no relevance. The raw support is built off the firmware for the sensor and that is unique to each model of camera. Even camera with the same sensor have unique firmware. Each time you start over from scratch.
It is worth noting that if Apple add raw support for the z8 it will only be for the current OS and later. They do not backfill to older OSes. It is also worth noting that they tend to offer support for one version of the raws and cameras with multiple raw formats often are at best, partially supported.
Alternatives: Nikon's own NX Studio is free, as is Adobe's DNG converter. Pro apps almost all use their own raw support and so already support the Z8 - things like Lightroom, CaptureOne, DxO etc. It's almost as if Apple, when creating an app focussed on their phones, don't really consider people paying $4k plus for a pro level camera and even more for glass, trying to process the raws with a giveaway consumer app.