Git branches start as literal files. When you create Feature-IB/test that creates the file .git/refs/heads/Feature-IB/test (later it might be "packed" into a packfile). If your directory is not case-sensitive then your branches cannot be case-sensitive because .git/refs/heads/Feature-IB/test and .git/refs/heads/feature-ib/test refer to the same file. See Git Internals - Git References for more.
On Windows you can set a directory to be case-sensitive, here's instructions for that.
MacOS is case-sensitive by default. If you should somehow find yourself on a case-insensitive MacOS system you can create a case-sensitive disk image and put your repository in that.
However, if you want others to work on your project it's best practice to avoid case-sensitive branches, tags, and filenames. It's best to pick another name for your branch.