A free mod for Mount & Blade II: Bannerlord, by Grisch.

Partyscreen Formation Assignments is a mod for Mount & Blade II: Bannerlord that adds back the ability to assign troops to form in the battle screen. It is a combination of the old Order of Battle system that is in the game and a new system that is implemented in the most recent versions of the game. With this mod, you can keep the current system and assign troops to form, but also remove the ability to change the units that are assigned to a particular formation.