Mouse lock isn’t something people constantly switch in and out of. It doesn’t make sense to use such a valuable button as shift to toggle it, especially when almost every game with a sprint key uses shift.
There really doesn’t even need to be an option to disable the toggle button if you move it to something that never gets used, like backslash or minus.
When the new camera/controls goes out, you’ll be able to modify this, as everything to do with shift lock was ported to lua. You’ll be able to change what key is used, or even change the icon. Also the EnableMouseLockOption bool under StarterPlayer will actually work with the new system, so you can turn it off for your game.
For the sake of consistency it would still be nice to have a default that’s more out-of-the way so you don’t have to relearn it whenever you go to a game that uses the shift key.