/Before modern medicine, nobody wondered when dead is dead./
Not true! Widespread fear of being buried alive (due to stories of seemingly dead people waking up from time to time) led to the invention of safety coffins, for example...
Or the old naval tradition that, when a dead sailor was sewn into his hammock to be buried at sea, the last stitch went through his nose in the hope that the pain would wake him up if he was still alive.
Is there actually proof of this? The OED thinks that it comes from boxing (where the bell signals the end of a round), and the first recorded uses of the phrase are in a boxing context.
That article could use some light editing, at least for that section. Right there in the section referenced it says that it comes from boxing:
"The "saved by the bell" expression is actually well established to have come from boxing, where a boxer who is still on their feet but close to being knocked down can be saved from losing by the bell ringing to indicate the end of the round."
Not true! Widespread fear of being buried alive (due to stories of seemingly dead people waking up from time to time) led to the invention of safety coffins, for example...
https://en.wikipedia.org/wiki/Safety_coffin