It would depend on the mechanics of the crash, but sure. Safety belts are inelastic, so any shock and subsequent deceleration experienced by the body at the end

It would depend on the mechanics of the crash, but sure. Safety belts are inelastic, so any shock and subsequent deceleration experienced by the body at the end of the belt's slack will be absorbed by tissue and skeleton. This is also why climbers use "dynamic" (elastic) ropes (when climbing, not necessarily for rappelling). Even a very short fall on a static line could be fatal.