Electronic – Can we route using plated through hole in a Multilayer PCB

Blind and buried vias add a lot to the cost of a multi-layer board, and are only used on high-density, high-performance systems. The increase in cost is because the layers have to be drilled separately, assembled, and then the holes are plated. Blind vias are sometimes back-drilled (the unwanted plating is removed with a slightly larger drill from the back) which reduces the cost, as the layers are stacked before drilling.

If your board house supports internal routing, you can use a 'wire' on layer 46 (milling) to make oblong holes. Basically set the width to 0 (or some small number) and draw the shape using the wire command. Another thing you can do is to create the oblong hole using overlapping drills spaced a distance apart (basically offset). That is the reason why some board houses disallow "overlapping drills", so you will have to check with your board house.