Sealed or non-sealed immersion cold-brew What's the difference (pros/cons) on brewing full-immersion cold-brew (resting at the refrigerator by night) hermetically sealed (closed mason jar) vs non-sealed (opened mason jar, Toddy, french-press, etc) ?
This is just a guess, but I suppose using a non-sealed container would speed up oxidation and deterioration of the coffee due to continuous exposure to atmospheric oxygen, just as storing the dry beans in an open container would. 

However, from my own experience, 12 to 24 hours of such exposure would not yield a perceptibly different taste. What's more important is that you store the finished product in a sealed container, preferably with as little air as possible (I use a water bottle). 

On the other hand, also in my experience, cold-brewing with a french press is more convenient, since one can simply push down the plunger and pour the finished product as opposed to lengthy fumbling with filters.