Star Trek: 10 Things We Now Know About The Temporal Wars

The Temporal Accords were put in place in the late 28th Century after the development of time travel. This was an agreement to restrict any such travel to merely scientific exploration. Naturally, it was abused almost immediately, and Lt. Yor became a known victim.

A Time Soldier, he was the first confirmed person to travel through not only time but dimensions as well. His point of origin was 2379 - in the Kelvin Universe, confirmed by Kovich to Dr. Culber. Specifically, he travelled from an alternate universe created by the arrival of a Romulan mining ship at the wrong point in history.

Yor began experiencing the effects of this travel. They quickly became so severe that his doctors asked for permission to return him to his original place and time.

The Interdimensional Displacement Restriction forbade this and, unluckily for Yor, by the 30th Century, it was considered 'ironclad' with all time travel technology outlawed after the Temporal Wars. With no other options, and no way to reverse the worsening effects on the Betelgeusean, his doctors then petitioned to euthanise him.

This was revealed when Emperor Philipa Georgiou who also hopped time and universes, began to experience the same symptoms. Her travel, being so much further than Yor's, was expected to be very much worse, prompting the crew of Discovery to attempt to find a way around the Accords. Perhaps milder symptoms were also endured by Prime Spock, Nero, the crew of the Narada and anyone else caught up in this sort of interdimensional criminality.