A more correct term would be open channel flow, flow with a free surface.
It’s really unclear to me.
Which underground and overground pipe flow stuff shouldn’t get waterway=pressurised?
A penstock (as you are referring to artificial infrastructure as well), whatever underground or overground, whatever its usage can be (hydropower, irrigation, sewerage…) is pressurised every time, aren’t you?
Are you proposing siphon=yes for actual siphons or inverted ones?