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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.

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    You need to consider the fork rake (offset) and how a different fork will affect the trail, which affects steering response. Beyond that my grasp of scients/art behind these measurements is not good. You may want to start by reading one of the meny discussion of head angle, rake and trail e.g. calfeedesign.com/geometry-of-bike-handling. As far as safety and strength - it does not matter, the differences in force based on a couple of degrees of head angle are minimal.
    – mattnz
    May 13 '20 at 2:37
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    @mattnz could you post that as an answer. It's okay to have short answers if they are correct.
    – ojs
    May 13 '20 at 10:09
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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.

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Yes, the fork you can use will be affected by head angle, in a different way than you think.

Have a look at this drawing: Picture from phil Gradwell on Flickr, https://www.flickr.com/photos/philgradwell/22395542694 CC BY 2.0

With a steeper fork you may have the front wheel hitting the frame (lower tube) or possibly striking your foot (right/left) when turning (left/right).

This is an answer to the question in the title.

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