On the topic of shoulder harnesses: Most early aircraft, including my own, do not have them. Guess what - you're not legal to put them in either without an STC or a 337 field approval, which is no easy task to get unless you're willing to spend serious money. You can't change anything in relation to your seats, no matter how bad they might be. For many aircraft, no such 337 precedence exists and not STC is available. It is illegal to add them.
A crystal clear example of how certification procedures and protocol can actually reduce safety.