Opticallimits

On the A9 (still issues in artificial lighting situations):
https://petapixel.com/2017/06/27/uh-oh-s...-surfaces/
And no, the A7R III has the same sensor as the A7R II, and does not have the reduced rolling shutter feature.

The problems in artificial light also exist in silent shutter mde of the D850. But there's an antidote for that: use shuter times which can be divided by the AC frequency, like 1/25, 1/50, 1/100 etc., the fllicker reduction doesn't help that much like these certain shutter speeds.