Hello! I'm trying to write a comprehensive answer here, but I'm struggling. In the American Civil War at least, packs would frequently be dropped before going into action, assuming they hadn't been loaded onto wagons for the march. The downside to this is that a unit might end the day miles from where they started it, without blankets, ground sheets, spare clothing, food, weapons cleaning equipment, etc. So, essentially, it was the commander's (probably regimental) choice - to drop packs or keep them on. Dropping packs certainly wasn't universal.