I use a mod that does limit Archer damage but doesn't completely negate it. That is good enough, maybe even a little too far. Archers now seem almost irrelevant, to the point where shock infantry is actually OP in the mod. Making high tier armor completely no sell it would probably be too much. Maybe a 50/50 chance or something.
Well, the aim I was looking for (before they said they were going to rework armor) was making it so melee was mandatory. Even giving bow damage a 50% chop made it so that you didn't really need to engage in melee, it just took a few more (not 50%) archers to kill them.
As for Horse armor I didn't say it wasn't used just not to the extent in the game, and certainly not without consequences, that is Armored horses running just as fast and tirelessly as those without armor. The Mongol Empire was atypical of steppe nomads since they had an industrialized base to produce such armor, and even then as Mr. Plano stated it was mostly things like leather or felt or cloth armor with metal over certain parts. The Strategikon mentions similar that only certain parts of the horse are armored. The horses that were completely armored weren't meant for HA's to ride on but for shock cavalry to use after enemy had become disordered. Compare that to the game where horses are galloping about to and fro with armor that is more protective than that available to people.
Well, none of the horse archers in Bannerlord use fully armored horses with the exception of Aserai Heavy Mamelukes. The typical set is the leather steppe barding that is 38 armor on just the head, neck, chest and loin/croup: steppe bandit raiders (sometimes), Khuzait horse archers, heavy horse archers, Kheshigs and Khan's Guard plus maybe Karakhergit. Bucellariis have very low horse armor (I think 18?) while the low-tier Khuzait troops have even less than that.
Horse armor not affecting anything helps cavalry catch up to HAs. The alternative of making armorless HAs actually faster would be pretty infuriating to play against. Any horse armored enough to last in pursuit is too slow to catch up. But if you take light cavalry, guess what HAs are exceptionally good at unhorsing with their arrows?
But overall it was very difficult for heavy Western armies to confront mobile steppe armies, and I don't think an effective tactical system ever emerged until Western states and armies became more consolidated after the 14th and 15th centuries.
The Mamelukes developed one in the late 13th century.