There are languages that use lowercased names for constants (Erlang's atoms) and uppercased Names for variables, which are immutable and quasi constants anyway. This is uncommon but many other languages use uppercase for classes (Person) and lowercase for variables such as Person person = new Person(). That last example is one reson for having case sensitive languages. I can't think of any language I used in the last 30 years that was case insensitive. Maybe some BASIC interpreters on home computers in the 80s? But not all of them had upper and lower case characters to start with.

Linux has a notoriously case sensitive file system, with trailing significant spaces (they are not special characters). Mac and Windows also have case sensitive file systems but that feature is turned off by default.