Why?

• Math methods usually return numbers, signbit returns a Boolean
• Math is more focused on proper mathematical operations, Number is more focused on implementation details (like the sign bit of an IEEE-754 float)
• Number methods usually don't perform coercion, which may be useful for this proposal
• It's more expected and consistent, it follows the "principle of least surprise"

One problem is handling non-numeric inputs, because it can't rely on coercion, so I suggest throwing an error, returning undefined instead of a bool, or switch to returning Numbers and return NaN