Sewer Line Opening - Makeshift Vent or Rodents?
I'm pretty sure that's rodent work right there
The scalloped, somewhat irregularly-shaped nature of the hole makes me think it was made by some rodent somewhere trying to avoid a nasty dental bill.  (Rodents need to gnaw to keep their front teeth from growing off into the other jaw.)  I would replace it with a hard PVC endcap or cleanout, as vents need to vent up so that you don't have nasty sewer gasses floating around at ground level.