Does headtube angle affect which fork you can use? If there is a fork designed for a slack headtube and you put it in a steep headtube, will the dropouts be affected by the different direction the force from the wheel is coming from? Will the wheel also be secure enough in the dropouts? 

Mainly asking about threaded steerers with quick release if that makes a difference.
No, dropout security isn't considered to be affected by the match between fork and head tube angle.

On bikes, concerns with QR hubs coming unseated from loads encountered is all about the relationship between the brake pad contact location on the rim/rotor and the orientation of the axle slot. The frame doesn't change that relationship.

It is true that one could contrive a situation with a forward-oriented disc fork pointing up to the sky once you had a slack enough head tube angle, such that yes, forces from the ground would be trying to unseat it. That does sound sketchy but I'd be surprised if it ever just happened by accident, because forks like that have a good amount of downward angle in the slots.