Why can't quads be triggers?

Hi!

I don't need to check if a rigidbody is inside or outside some collider. I just want to determine is a rigidbody forward or backward passing through a plane.

I thought using quad as a trigger was the perfect and most suitable way to achieve that.But I was wrong. As you see, Unity doesn't allow me to mark non-convex mesh( including single sheet quads ) as trigger. Making a single sheet quad mesh convex is just meaningless for me.

Why is it so hard to achieve such a simple and primary idea? Is there any way to achive this?

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