Well, I guess I understood what was the problem of shortage of Imperial Chargers after 1.5.7:
In Imperial villages breeding horses they produce many types of horses (idk why and this is weird): Midlands Palfrey, Imperial Charger, Sturgian Trotter, Sturgian Hunter, Saddle Horse and Vlandian Courser (and also they have Primary Product description: Saddle Horse)
I also checked horse breeding villages of other cultures - they produce only horses of their culture (+Sumpter Horse and Mule).
Battanian villages - only Battanian Pony and Battanian Warmount; (Primary Product description: Battanian Warmount)
Khuzait villages - only Steppe Horse and Steppe Warhorse (Primary Product description: Steppe Horse)
Aserai villages - only Desert Horse and Aserai Horse (Primary Product description: Aserai Horse)
So, my guess is - Imperial villages producing Sturgian and Vlandian horses have now much less resources to produce Imperial horses and that's why it became so hard to find an Imperial Charger for a sale in the game.
@mexxico could you, please, check if it was done by some mistake?