This is just my experience, but when we were "engaging a hostile force" in an offensive attack, we dropped everything but our weapons and ammunition.
That means backpacks, canteens (we usually carried at least four), and any other non-essential stuff.
The helmet, flak jacket, ammo bandoliers and rifle were more than enough
weight to carry doing squad-rushes up a hill in 90+ degree temps, while trying to direct accurate fire at the enemy.
Dropped gear would be brought up later by those who did not actually participate in the "assault", but gave us supporting fire. By that I mean
mortar teams, MG teams, HQ people, and anybody else who wasn't
doing the "John Wayne" up the hill.
Most of the time we got our gear back, sometimes stuff got lost and we never saw it again. But if you survived the action, it didn't matter too much.
Gear could be replaced.
RVN, 1969, 1970