I don't think any camera guide mentions it, most manufacturers remain mute. It is discussed regularly and typically pointed out during new camera reviews. If Sony were trying to keep it a secret, they wouldn't let their shills say anything. There's no chance of keeping anything like this quiet these days, someone is going to spill the beans. I suspect they, like other manufacturers, don't discuss it in print for some technical reason. I have discussed it with Sony reps at local camera events on several occasions, it's no secret. In fact, they like to tell you about the advantages in practical application.  One thing I noticed in the chart above. The DR chart shows a smaller gain at ISO 400 than the read noise chart. This one is more accurate for noise, and you can see that ISO 400 is actually similar to ISO 126. That's a huge advantage. If you check this chart for your A7 RV you'll see that ISO 320 gets you down to between 126 and 159, seems plenty usable to me. 

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And a different look. The shadow improvement charts show where the shadows are improved for DR, and how much. In the A7 IV there's about a 2-1/2 stop improvement at ISO 400, and then no additional improvement at all until you get to ISO 56K, where Sony applies in-camera NR. According to the primer on this chart, there is no degradation of noise along these flat lines, like from 400 to 51K. Not sure I can agree with that, but I can certainly recover image into ISO 50K in testing and shoot ISO 16-20K successfully.         

It is 320 for stills on my A7R5 so it’s 5/3 stops. I have used it on occasions but it’s a bit low to be any huge deal. It is much more use in video where it is 2500. Have a search for any of those terms you mentioned above in any current Sony camera online guide & I doubt you will find anything. I was at an event yesterday where there were two Sony reps & they wouldn’t discuss it even though one of the presenters mentioned it when talking about his A7S3.

Yes, I would always wait a week before installing just to see if anyone has problems. This is good advice for any software updates to be honest.