Framing Advice - Removing Shower Opening
With your joist direction following the same without support it is probably built for a door. With a door and no header the wall may wobble a bit as many shower doors are glass and heavy. I was taught to always put a header in for a door for that reason. If you are not going to have a door and want it open to the ceiling I would do it. Depending on the depth and spray a shower curtain can span the opening with an expanding rod if needed. Opening up the area (this also held ventilation and may reduce mold growth as your extractor fan can pull the steam out.