Flight global archive, 1983 - "Thai Airways ... cleared of recklessness and negligence in a case brought by a passenger injured in clear air turbulence." I believe that was actually a judgement on appeal, overturning a previous decision.
Interestingly, "
Without proof of negligence, the injured passenger could
claim compensation for his injuries up to a limit of 250,000 francs". If I remember correctly, the "Warsaw Convention francs" would be of the French gold franc variety, and complete with a defined gold fineness too. I believe in the UK at least, government "Statutory Instrument" equivalents might have been applicable.
1983 | 0880 | Flight Archive
However, the message has always been clear, "In any event, the passengers had been warned before flight to keep their seatbelts fastened whenever seated."
Edit: I had some trouble aligning the Flight archive link, but eventually got there!