You can call me a dick (because I did act a bit like a dick which I'm sorry for), but please, I'm playing 18-21 cav right now. Before that, I used to play as 15-30 assassin guy. And before that - as a 24-18 polearm guy. So I'm not trying to convince you just for my own comfort. And my current build is quite OP by the way.
My point is that restricting isn't equal to balancing. If armored agi is OP, it should be nerfed instead of being restricted. Otherwise it would be unfair since STR builds are free to use light armor as well as heavy one.
The game is very much balanced around agi atm, with little incentive for players to take more str than around 18. Every build favours agi really opposed to str. Be it melee, cav, archer, xbow, thrower. What do you get by going over 18 str compared to what you get with agi? We definately need str limits of some kind on armour btw. Thats the only negative trait to wearing armour on a horse.
Agi is too good atm for every class. It adds to melee damage, lets you use ranged, use cav, makes you way more efficient in melee. They either need to buff str or give people a good incentive to take less agi. I'm not sure if str requirements are the answer, but it would at least put restrictions on ranged and cav which is fine for balance. Some classes do need to be restricted, its not always possible to simply nerf for each class. Agi melee might suffer, but tbh melee aren't the ones who benefit from high armour the most anyway. Melee need to be fast to catch people because of their short range. Cav aren't affected until dehorsed, and then they just need to soak up damage or run away. Ranged always try to put themselves at a distance and can start to kite way before the negative trait of the armour will impact them too much
I have my wpf split 3 ways and I still reckon just because I have 8 athletics my build is better than my old 24/18.