Welcome to the forums
Yes, that horizontal line is an intentional part of the galaxy map's graphics. It's sort of like a "scan line" from various sci-fi-ish data readouts, etc.
Reduce Visual Stimulation does indeed disable that. That whole option is basically for my wife who gets migraines from excessively flashy things, but she loves to play AI War. Hence the option. But it does disable almost all the animation in the game.
It would be possible to add a separate toggle that only disables that one horizontal scanline thing, but we normally try to avoid such gratuitous additions of toggles to an already huge set
I might be able to help you, though; what exactly bugs you about the scanline?