A nice trick is to light a fire in a mine level above the working one, the fire updraft and fumes do escape from a vertical shaft purposely built on top of it, the updraft sucks air form the levels below.

You need to arrange it with doors so you can force the fire to suck from the area you like and the outside air to be sucked in the level you want.

But that’s a method doable even at Stone Age tech.

Idk if gas pockets were common, as, ancient digging was superficial and by the time you get the tech to dig further down I imagine they got the tech to protect themselves. In between these two era they just died. I mean, there’s plenty of mine horror accounts from the 1700-1900. The whole industrial era was a gigantic trial and error thing. I remember industrial ventilation devices like hand operated, animal operated, and later steam operated air pumps, and acetilene lamps made “flash proof” to an extent. Problem is, if you get to a gas pocket, even if it doesn’t ignite, you are still really dead. There is no way to replace that much gas with fresh air in time. Having a bird that dies earlier than you is a good warning system, but to escape you have to climb a lot and climbing with little oxygen is not an endeavor that is famous for its success rate. To conclude, early ventilation systems were more about getting enough air to breathe and evacuate the lamps fumes.