Sony could have included the a9 AF in the a7rIII or a7rIV but didn't.
Their marketing is obviously deciding to segregate capabilities based on target market segment, and I think that the main target segment for the a9 and a9II is clearly pro sports shooters, not wildlife shooters.
I could be wrong... but I don't think that I am.
We also know that Sony will not announce an a9II until November at the earliest since they won't want to blur the marketing message associated to the actual availability of the a7rIV with a competing launch.
This means that you won't be able to spend money for at least another 4 months, maybe 6 if you wait for actual a9II real world test results before buying.
Cheers,
Bernard
They couldn't have included it on the A7r3 because that camera uses an older sensor design, unable to support it. The A7r4 uses a new sensor, which has been stated to include real-time tracking by Sony and which appears to match the A9 in capability in at least one report.
Sports photographers make up a tiny minority of A9 shooters. They sell far more to wedding shooters, photojournalists and other photojournalist types (e.g. event, live music and live theatre photographers), rich amateurs wanting the newest and fanciest gadget out there, and will have a growing chunk of wildlife enthusiasts now that they're expanding their supertele range. Sports photographers and the Olympics may be their showcase, but are far from their biggest and most important customers.
Besides, Sony, unlike Canon, doesn't have much of a history of segregating capabilities for the sake of market segmentation. Any technology or new capability can show up in any camera, depending on when it's ready. Animal eye AF first showed up in the A7r3, when the A9 is more of a wildlife camera. The A7r3 works better with mechanical shutter than the A9, when the A9 is more likely to be used in roles that actually call for it. The A7III has better AF and a more advanced (if lower resolution) sensor than the much more expensive A7r3.
Besides, they already have a suitable new sensor, and it's 36MP (it's unlikely that a premium version dedicated to Sony's flagship action camera will have less than this sensor, which is being sold to everyone). The A7r4 went from 42 to 61MP. 24 to 36MP is a similar increase, and would reflect the increased data bandwidth and improved sensor design over the same period of time.
I have no need to spend money on a new body just yet anyway. The A7r3 works fine as it is - almost anything I shoot that can benefit from 61MP can also be stitched, for even better final output. If the A9II has at least 36MP and even better AF and high ISO capability, I'll be seriously looking that as a do-everything body to cover both landscape/cityscape and action work. Doubly so if it has a global shutter, but I doubt that's ready yet (although Sony has surprised me before - I expected A7r3-level AF for the third generation of full-frame mirrorless cameras, with A9-level, SLR-matching AF only coming in the next generation, i.e. those being released now). But I'm more interested in new superteles, a better 70-200/2.8 (the current one is a disappointment which I refused to buy) and some super-sharp f/4 zooms, which Sony should develop now that the f/2.8 zoom lineup for the journalist/wedding crowd is complete.