It would appear that in OneUI 3 we are no longer able to change the shape of icons on our phones. It would be really nice if we could bring this feature back. Another useful addition in this area would be using icon packs from the play store (there aren't any I like in the Samsung themes). I used to have a OnePlus and I really appreciated the ability to customize every app icon individually instead of skinning the whole OS.
It's only a half step solution. When you use theme park to change them they have a disgusting white drop shadow that you can't control and 90% of apps don't scale properly, they're just placed on a white background. Camera app for instance.