Not being able to assign troops and lords in the party screen is intended but the reasons are different.
Being able to assign troops to different formations would break the saved values and how the percentage sliders/filters work. If you could assign a horse archer to an infantry formation, you couldn't get him out of the infantry formation in current implementation of the OoB. Zeroing the Infantry slider in that formation would still keep the horse archer you put in there. OR we disregard the horse archer assigned there are, move him back to a horse archer formation when the slider value changes. Formations would be a mishmash of troops and it's not possible to represent that with sliders.
If we add all 4 troop type sliders to all of the cards then one of the problems is, "when do we remove the troops assign from party screen?". Player changed the filter, do we remove them? Player set the slider to zero, do we remove them? I'm not even going into AI problems with having 8 different mishmash formations.
Now one way to overcome that can be locking each formation's troop type from the get go and not letting the player change them. Formation I-II is Infantry, Formation III-IV is Ranged, Formation V-VI is Cavalry and Formation VII-VIII is Horse Archer and you can only assign a troop to a formation that is related to their equipment. But we didn't want to restrict the player to these pre selections.
All in all, assigning formations to normal troops in the party screen is conflicting with changes made in OoB that's why it's removed.
Now lords/companions are a different story. Since they're more fluid with their equipment and they're unique, they don't have to conform to the formation types. We can handle them in a different way. This doesn't have to be in the party screen, we can add a separate UI in the formation cards that you could use to put unassigned heroes in that formation as troops. We can save the assigned formation of the hero and not let them be affected from the sliders. I am, personally, not against this and would love to bring it up internally. That's why I asked for a suggestion post,
here. If you feel the assignment of heroes to specific formation as troops, not just captains, would be a good addition feel free to leave a comment in that post.
