ZDoom

VisibleStartPitch
VisibleEndPitch

If I knew how to use these and other similar things like this, I'd be able to make decal like actors that can spawn on floors and ceilings from things like gibs, bullets, and other stuff... It'd be like DECALDEFS, but for actors and you can actually see what they would look like, Also +FLATSPRITE and +WALLSPRITE, how would they stack against other actors, would I need to change the z offset for the sprites or would I need to use A_Overlay, I don't want to use modeldefs and md3 files and other stuff like that, because I want my projects to be as simple as possible.

I'm trying to understand what you're asking and what you're trying to do.

Most sprites are drawn billboard style meaning that they always drawn perpendicular to the observer's POV, so you will never see them from any angle except head-on.
A Wall Sprite does not have this behavior and can be viewed from many angles, like a wall texture is rendered.
A Flat Sprite is also not a billboard sprite, but is drawn in the XY dimension instead of the normal XZ dimension, so it can be flat on the floor or ceiling. It can also be pitched like a sloped floor or ceiling.

For an example of how these can be used take a look at my Look Cursor and notice how you can point the cursor at different walls and floors and ceilings and even slopes and the sprite always matches the angle.

Is that what you're trying to do, create monsters that stay stuck to the walls/floors/ceilings? Kinda of like Zelda Link Between Worlds?