Add a "Guard" movement key that's separate from Sneaking.

The only difference between the two is Guarding would let you step off ledges. If you are trying to advance against a hostile ranged mob like a Stray or Skeleton, it's a nuisance to let your shield down just to step down one block to continue to advance (and they nearly always get a shot in, which defeats the purpose of having the shield).

The security against accidentally sneaking your way into a fatal fall down a ravine is a great thing, but when using a shield it becomes a liability. And if I accidentally Guard my way off a cliff then I should've just picked a better hotkey to use.