I use my lap belt all the time with no issues, some of the cars did not even have a shoulder belt. My 69 fish does but stay strapped up. Until recently I was not required to use one at all in my old 74 truck. State laws vary so hard to say.
Same here. I only wear the lap belt in my 68 Barracuda or 69 Dart. Hell most states don't require school buses to have belts, at all! Florida only requires lap belts and only after the year 2000 they were required when new.