Questions about trapezoids should be tagged with geometry and quadrilateral. That's specific enough: there are only $29 $ questions in the latter tag. Splitting it further into squares, rectangles, rhombi, trapezoids, parallelograms, and irregular quadrilaterals would be more harmful than useful.
Why harmful? Because more tags means more mis-tagging and subsequent re-tagging. When one proposes a new tag, it's easy to imagine it being used to neatly classify questions. The actual use... can be illustrated with a picture: ISS mockup vs reality.
A randomly chosen question about a trapezoid is more likely to contain the word trapezoid than to be tagged correctly. The text-based search will beat the tag-based search.
And I'd guess there are no users who love trapezoids but hate parallelograms. So the tag would not be useful for filtering.