The smallest bend radius that can be used on a membrane switch tail will depend on the type of design. Deciding factors will depend on whether or not the user interface is constructed with one tail/circuit layer or dual tail/circuit layers, and where the bend is located.
Custom low-profile keypad assemblies used in OEM applications incorporate snap-acting dome switches. Dome switches are a type of keypad assembly switch intended for very long cycle life and years of trouble-free use but are intended for logic-level switching, not power switching.