In itself it does not matter much whether labels are case sensitive or not.
Making it a user setting is a horrible Idea for reasons SembazuruCDE explained. It would create more confusion then it solves, and it will break portability of projects, and for these reasons I’m convinced it will never be accepted by the people working on KiCad.
As I showed earlier KiCad-nightly V5.99 already has an ERC rule built in to check for inconsistencies in upper/lower -case use.
Schematic Editor / Edit / Search and Replace can be set to match case:
… but it is not a very fine-grained search function.
Schematic Editor / Edit / Edit Text and Graphics Properties has much more powerful filters, but it can not modify the content of text strings.
Having an option to for example set all labels to uppercase only seems a legitimate request.