Positioning. Cav cannot bump you if they don't have enough speed.
If foot unit is mindful of his positioning he cannot get bump hit. Tbh I cannot remember the last time I was bump hit.
it is good that you do not get bump hit alot, as it is very frustrating to play against if there is no defense to it (in open ground as you stated if you do not have a spear or 100% accuracy bow)
I do get cav bump unblock-able struck alot, but only by the same few very talented people (the same people that can kick you 100% of the time if you are a really slow class like heavy cavalry)
I think you overestimate the safety of walls and other "positioning" elements because cavalry can still horse bump unblock-able strike you when you are against a wall, they will just be more vulnerable after bc they will be reared/available to punish afterwards bc they are stopped by wall which does not matter if they kill you with the unblock-able horse bump strike anyway.
the only "positioning" I see that is defensible (besides an elevated position etc) is a tree or column where you can circle it endlessly to avoid the cav-bump
I just do not believe this is a good addition to the game because it is not fun to play against (it is fun to do to others though) and would like to remove it from the game to encourage more dismounted melee instead (given the removal of the movement penalty on dismounted cavalry, so that cavalry can more freely engage in dismounted combat rather then rely on unblock-able strikes)
Cavalry would still have the unblock-able couching mechanic(without shield) but at least that can be chamber blocked (couching should not be a able to lance through the horse's head on the left side of the horse)