As near as I can tell, suspending a game after it began was unprecedented before last night.
Games have been delayed after starting (power outage, lightning or other weather related reasons), sometimes for hours, but I don't know of any other game that was suspended indefinitely before the first quarter ended.
Kudos to the Bengals, their fans and the city of Cincinnati for recognizing the seriousness of the injury. I've never seen a stadium filled with 65,000+ rabid fans as quiet as Paycor became once the ambulance came onto the field.
Here's hoping young Mr. Hamlin suffers no lasting debilitation.