[2020] MOVED: Were curved corners and curved edges removed from game ?

Glass in hand, hold R(or whatever your reload key is if you remapped it), select shape menu. Should be in there. Note that other objects also have shape menus like wooden furniture, metal furniture, wooden frames, rebar frames, wet concrete blocks, etc- keep an eye out for the icon.

If a shape isn't available in the shape menu, remember to try different materials where it might exist. If you still have no luck, creative mode is an option to get what you want.