I don't think I've ever seen non-round manhole cover. In all exUSSR countries they are round.

Are you talking globally or just USA?

I guess it varies by country which I should have acknowledged. I was talking UK and EU and Africa which I am most familiar with, although I have also been to the USA and Asia and seen manhole covers there. You certainly get round manhole covers, but there are far more rectangular ones. Eg this kind of thing https://www.drainageshop.co.uk/polydrain-inspection-chamber/...