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I assume that by 'obscure' you mean 'outside your-state, USA'. Here in the UK, rectangular manhole covers are the overwhelming majority.

There are many cities where manhole covers are overwhelmingly rectangular. I'm typing from one right now: Rome. SPQR!

I used to live in a town in the US where many of the manhole covers were shaped like a D.

I think the point is that a circle is not the only shape that can't fall into it's own hole/fitting, so it's the combination of this fact along with other factors that make a circular cover popular.

https://en.wikipedia.org/wiki/Manhole_cover#Circular