Iirc Ubuntu names their home files “Downloads”, “Documents”, and so on. Same with windows (there are a lot of uppercase letters in windows files). I’ve had issues with Cargo.toml before. And not just cargo, many config files use case to signal priority (so if both Makefile and makefile exist, Makefile will be used (or other way around)). Downloaded files are a gamble. Files created by user input (so for example if I wanted my user to be “Calcopiritus”, my home would be “/home/Calcopiritus”.

Uppercase letters might not be common in filenames, but they are not nonexistent.