I understand that these Living History flight operations operate under a waiver, and many FAA Regulations are "relaxed" for these operations
The nature of the airplane probably results in the need to waive some operational requirements which are normal for passenger carrying flight. These flights are just different, and still in the public interest. I would expect that the B-17 is characteristically unable to meet some of the "normal" requirements ( I opine exit and exit path design requirements, for example). That's all the more reason that requirements which
can be met, seat belts first among them,
should be met.
A seat belt with shoulder harness, then a life jacket saved my life once, because I was wearing both properly when the accident happened. And, on the other hand, I have refused certain operations in airplanes which were not equipped with a shoulder harness.
I hope that there is a continued interest of the importance of seat belt use. I have occasion to ride jump seat observer from time to time in an older aircraft, in commercial operation. It surprises me how often the pilots wear only the lap belt, when the shoulder harness is easily available, and perfectly suited.