Easy way to seal an old plumbing vent
Fernco makes rubber caps that you would put a stainless worm-gear clamp around.  Take your outside diameter measurement to a plumbing supply house.. What your have is a main drain and cleanout wye.  As suggested by dmoore, future plumbing may be connected, though its not a toilet flange.  Its likely the point of exit for the drain system (from the house), as it traverses to the sewer (or septic), to allow the drain to be snaked out, if obstructed.