Why would a homeowner cut this? My only thought is the vent was plugged and this was easier than cleaning it?
Do you have a larger photo of the whole furnace?
Not really sure what the reasoning is for it. It is done in an unprofessional manor so I would recommend corrections by a licensed HVAC contractor.
Looks like a fresh air intake to me. It shouldn’t be big problem unless there is not enough combustion air in the space the furnace is, but like others have said I would call it out. I would say “fresh air intake is disconnected, recommend contacting a licensed HVAC contractor to repair as necessary”.