It did have sewer smell coming from it until he covered it. An open vent for future use would just be letting air in and out. It has to be connected to an active sewer line and most likely a vent as he hasn’t said anything other than gas comes out. Capping it may have no effect or it could cause a problem depending if the line it’s connected to has another source of air. It could slow down flow in some drain or cause gurgling. Last the OP was here he was going to report back on what closing it caused.
Most of the radon vent lines I see around here are bigger than 1.5 PVC I don’t know what is normally used.