The two cameras are not completely synched in features and firmware, even apart from differences that may be based on physical differences between the two cameras. So either one might get a feature first.I got my Z8 the day that they were released. I debated for a while previous to the announcement as to which I thought that I would get. The Z8 was said to be a mini Z9 but that it had all the features of a Z9 except the grip with the big battery. So I bought the Z8... then within about a week, the Z8 no longer had all the bells and whistles of the Z9. And for most of the past 10 months the Z8 has not had some of the better features. If I had it to do over again, I would now own a Z9. Just my 2 cents!
The Z9 got bird ID first; the Z8 got it later.
The Z8 got cycle AF areas first; the Z9 got it later.
The Z8 has pixel shift; the Z9 does not have it. Not a particularly useful feature for wildlife work, but potentially useful for other work like landscapes.
For now, I have one of each. Not because of firmware differences. But there are times I want a smaller lighter body.