The bottom line is that, as it was pointed out in the limitations section, it's not required to be on during T.O. or landing. In fact for the first few years that the aircraft was in service, the limitation was yaw damp was required to be off for takeoff and landing because the rudder boost was required to be on for takeoff and landing and the limitations prohibited the yaw damp and rudder boost from being on at the same time. So after each TO and before each landing the pilots were required to turn off one system before activating the other system. I've been flying the Lear 40/45 as PIC for nine years and the few times that I forgot to turn the yaw damp off before landing it was a very uncomfortable sensation to feel the rudder pedals fighting all my inputs. Very disconcerting to say the least. Works much better in all situations to have the yaw damp off for takeoff and landing, especially in a crosswind situation.
So, in response to the "It must have been certified with the yaw damp on for takeoff and landing.", the fact is that it was originally certified with the yaw damp off for takeoff and landing.
Hope this was helpful.