As long as the vertical pipe above it isn’t plugged, you should be fine to just plug that offshoot. It’s part of the vent stack for that drain, which is why it smells like a sewer. Presumably there was something else plumbed in there to drain out at somepoint, but has since been removed.
Thanks for taking a look and the advice. I just went through our old demo pics and noticed the small pipe runs up and joins the primary vent stack. Now I can plug it and be done with that stench.
Think an expansion plug or should I try to chop the pipe and cap it?