What is the point of setting an upper limit in Auto-ISO?

If you are worried about the light changing and are too busy to look at the histos, why not just set the ISO a bit lower than you'd normally set it?

Yes, I agree the histogram and blinkies (in my case with the 90D) is better, especially shooting raw, but after setting the optimal exposure**, where ISO lands with Auto ISO is not a big deal for me as long as important highlights are not clipped. I set the final image lightness in post.

* exposure - amount of light that struck the sensor per unit area while the shutter was open
** optimal exposure - the maximum exposure* within dof and motion blur requirements without clipping important highlights.
*** under exposed - more exposure* could have been added with the DOF and blur constraints still being met without clipping important highlights.

Bolding is mine. If you let the camera set the ISO, you have lost precise control of the relationship of the highlights to clipping.

But only if I ignore the histogram is it not?

I take a shot, look at the histogram and if it doesn't show highlight clipping then the ISO the camera set is largely irrelevant since I have set optimal exposure**, isn't it?

* exposure - amount of light that struck the sensor per unit area while the shutter was open
** optimal exposure - the maximum exposure* within dof and motion blur requirements without clipping important highlights.
*** under exposed - more exposure* could have been added with the DOF and blur constraints still being met without clipping important highlights.

From last night, an out of camera jpeg with a tiny bit of editing resized vs a raw with a trip through Pure raw. Floating ISO. Pretty darn close for simply letting the camera do 90% of the work.

You shot it at 1/100 of a second @ 24mm, and used a flash -the great subject noise eliminator. Odd choice of settings

You can't win much by setting ISO so that the image highlights are close to clipping. However, you can lose big if it's clipped. I thought you were trusting the camera to adjust ISO as the light changes. If you're going to look at the histogram all the time, you might as well change the ISO yourself.

Why let the camera choose a setting that can't help you much but can hurt you big time?

On my 90D I have never seen highlights clipped due to Auto ISO when the camera set ISO above base.

If anything, in my experience the 90D is over cautious with Auto ISO.

I would think that depends on the metering used. Does 90D have only one kind of metering?

No it doesn't.

Then you are saying that regardless of the metering selected, Auto-ISO does not clip highlights. I do not believe that,

You are making things up. Post a link to where you claim I said that.

You asked

and I replied

You said that

I read your claim that this is regardless of the selected metering.
If 90D had one metering (a special do-no-clip-highlights-in-Auto-ISO, or a strong highlight-weighted one) then I could believe it ... kind of.

Then you made an incorrect assumption by assuming I have used all the metering modes. I didn’t mention metering modes at all so if you were in doubt you should have asked instead of assume.

One might set an upper limit on ISO to alert themselves to some other semi-auto guardrail that they have in place and forgot to remove. Like shooting aperture priority with a high minimum shutter speed for a sporting event yesterday. Then today you’re taking indoor pictures and you’re at ISO 20,000 and 1/1,000. Maybe bumping up against the ISO cap gives you a blinking setting reminding you to remove the minimum shutter speed.

Which metering mode did you use that does not cause any highlight clipping in Auto-ISO?

It might be silly approach, but it worked for me, kind of...

I set the upper limit of ISO speed to not get noisy images. With given exposure and given maximum aperture and ISO speed I didn´t get the brightness, the camera reminded me with blinking, that the image is unobtainable by my standards. I either had to add risky shutter speed, or add light, or wait for light, but at the momentary circumstances, things went red and I knew I could spare my battery and efforts.

It was not the wisest tactics, because now I might miss some photos, which I could delete if I didn´t like them, but oh well, no failed shutters and no biggie. :-)