That is all nice and correct in theory, but that is not how most of us shoot in the field. I use manual exposure in auto ISO mode a lot to shoot wildlife. I base my exposure by the in-camera histogram in the viewfinder and the setting level for zebra- stripes in my Sony A1 viewfinder. I adjust shutter speed and aperture to " expose to the right" to maximize the exposure. So ETTR is important no matter what ISO you use. I think all the discussion that restricts ETTR to only base ISO just confuses people. It is valid at all ISOs as long as one remembers that the"signal" in signal/noise ratio (or the "Exposure" in ETTR) is dependent on shutter speed and aperture.
I know I can get away with more underexposure at base ISO, because I am looking at data that is not being artificially amplified by a higher ISO. However, significant underexposure when one happens to be necessarily shooting at a high ISO can be devastating. So to me that means ETTR is even more critical at high ISOs.
I do dabble a bit in real world wildlife photography too
Here's an example of what I wouldn't do. Let's say I had a deer in a field, all middle tone, and it was dim with a proper exposure of 1/250th at F/4 at ISO 6400. This is as slow a shutter speed as I am willing to use and my lens is wide open - I've let in as much light as I can. If I shot it at those values, when I got back home, I wouldn't adjust the brightness at all ( I kind of get the impression you think I'm advocating underexposure and I'm not). In that scenario, I'd probably have to shoot at ISO 25,600 to push my histogram all the way to the right. I wouldn't do it. I'd shoot at ISO 6400 and get the proper level of brightness. If I shot at 25K and brought it back down at home by two stops, the result would look the same as had I just shot it at 6400 in the field, BUT I'm flirting with clipping and I my color fidelity may not be as good either. I guess my question is, in that scenario, would you shoot 6400 or 25K?
I do agree that if I have something in the frame that should be close to white that I'll adjust the histogram / zebras to a point just before clipping, but that's not really ETTR, it's just exposing properly. Sometimes a proper exposure has the histogram to the right. On the other hand, ETTR is generally done by deliberately overexposing* to increase dynamic range and then bring the brightness back down in post. If you already have all the light coming in that you can muster using shutter speed and F/stop, there's really no good reason to crank up ISO to make the image overly bright and then bring it back down. It's exposure with extra steps. IMO, it's better to just shoot at the proper ISO so no brightness adjustment is needed later on.
*Technically, ETTR is just exposing the brightest highlight areas you want to preserve to just before clipping, so there are times an underexposure is necessary for ETTR. However, most of the time an ETTR image looks too bright