I wouldn't be worried about it. I had a dealer tell me once that my car was unsafe to drive and should be towed home when I refused an expensive ABS repair. I asked why it was unsafe to drive, as I thought it may be worse than what he actually said next. He said I could lose control if I need to emergency brake while avoiding an accident. So, I asked if the braking system was still functional, and he said it was, but ABS wasn't working and shouldn't be driven. I've been driving before airbags, before seatbelts were required, before traction control, before 13 cup holders in every car, and before head rests actually were behind the head.
Anybody sitting in that seat should be fine.
