Dat sexy narrow eyes, why no special tag for this?
Flat eyes could be a tag, but there's lots of different kind of eyes that are sort of flat. Stylistic choices like the shape of the eyes, the thickness of shoulders, or the width of the skull can be hard to tag and are usually the most easily forgotten tags.
Also, flat eyes is more of a western style thing that is particular to specific artists. I wish I could name an anime or artist on this site that draws eyes this way, but I can't think of one right now.
I was just going to say that at first, but I personally never liked using the tsurime/tarame/jitome tags (not to say anything against their use). I think they produce too much mixed results if put in search. These eyes, for example, looks nothing like Reimu here. I guess it depends on how specific you were trying to be.
Nnm...Wh-Wh-Wh-What's wrong, Reimu!?A once in a lifetime opportunity...!?
What will Marisa do!?clonkMm... I think I'm a little...Party...a little drunk...Wah!