If websites can detect that you've disabled favicons, then you are easy to track between all websites because you are very unusual.

I don't think that's true. You'll just look like someone who already has it cached.

It depends on how the browser rejects favicons? If the browser reports the icon is already cached, I agree (assuming the reports are indistinguishable). But maybe it just never downloads the icon, for example.

> If the browser reports the icon is already cached

Browsers don't do that.