Ah, I see. Thanks for the prompt reply, and correcting my misconceptions!
Only CSS.supports("aspect-ratio: 1") is false. I have to admit it puzzles me that you would need to detect that, especially since as I went looking to see what styles rely on that, I found quite a few places in the CSS with @supports not (aspect-ratio: 1) {/*...*/} blocks and the like, which would seem to explain why the sites’ appearance is fine despite lacking aspect-ratio support. (I’ve never noticed any visual problems anywhere despite using Discourse extensively in both PM and in other browsers.)