Don't forget that for many aircraft, the both the Mach Trim and Yaw Damper reside within the Autopilot (aka Flight Control Computer or Flight Guidance Computer - two of which are the norm on most non-CAT III aircraft). Some aircraft can not dispatch without at least one Yaw Damper operative or, if they can dispatch, the limiting IAS essentially prevents normal operation. Similar limiting performance can also be true for the lack of Mach Trim.