Pain on bottom foot when freewheeling for a time in a gravel descent
It's generally a good idea to change position regularly (at least slightly) during any ride. If you get used to just one position, your body learns to keep that position no matter what, and it'll do that by stiffening up if the bumpy descent tries to shake you out of it. But stiffening is the worst thing you can do in such a situation: it'll cause the stresses to concentrate right into the most exposed joints and muscle sections, instead of being able to harmlessly distribute it over the whole legs. What's more, having the left foot always down is a recipe for pedal strikes in left turns. A pedal strike at high speed can lead to a very nasty crash. So definitely get rid of the left foot down habit.
One easy way to train flexibility is to slowly back-pedal (on the straights; in corners, always keep both pedals level or outer foot down). But I wouldn't focus only on the pedals – the whole body position influences the forces on hands and feet.
It helps a lot for body flexibility to have the saddle in a really low position on descents. A dropper post is super useful for this reason (now standard on most mountain bikes). With a low saddle, you can stand up without needing to stretch too high, move your body weight far over the rear wheel when it gets steep, balance the bike side to side underneath you etc.. And when sitting down, neither leg will be stretched at all, so the knees have plenty of freedom to move.
Of course, MTB suspension also does a lot to make rough descents more comfortable, but it really isn't needed for gravel. It does make sense to use relatively wide tyres and low pressure though, and some bikes a more compliant than others. Steel or carbon fibre tend to be better than aluminium in this regard.
Perhaps counter-intuitively, in my experience it also helps to clip in, because then the legs can relax even more without risking to slip off the pedals. So clipless pedals &amp; shoes (I recommend Shimano SPD) are perhaps a good investment, if not a dropper post and/or suspension fork.