Linux

OK, you're going to have to be specific about your technical claims here. Which part of unicode support on filenames prevents case insensitive filenames?

I mean, I don't know why you made me try, because I do use non-English characters day to day. But you made me try, and sure enough, you can absolutely mix and match alphabets and cases in NTFS and still have it behave case-insensitively. i.e. делоinsensitive.txt and делоINSENSITIVE.txt can't be placed in the same folder and are parsed as the same name.

So if you have some bit of nuance about why you "cannot have" the feature that clearly works right now in front of me in my computer and has for ages I'm all ears. For a completely impossible implementation of a clearly useful feature it sure seems like it is actively supported in most of the FSs currently in use.

EDIT: Made me go check because now I'm curious about the edge cases you guys keep claiming are catastrophic. Putting those two files in a case sensitive Samba share an opening it in Windows keeps the mix of characters but a suffix gets unceremoniously appended to the filename of one of the files. That's probably a bit of a mess in some circumstances if you don't know it's coming, but there you go.