You can remove the outer casing with a sturdy flat head screw driver and a hammer. Hammer the screw driver in sideways (horizontally) on the outer casing until the screw driver hits the nail in the middle and then pull the screw driver out and there will be a gap you’ve created in the outer casing. Now you just have to hit the screw driver on an angle on both edges of the opening you’ve created and the outer casing will break off. Repeat these steps for however many anchors there are and you’ll be able to remove ramp.

Hard to tell for sure from the photo but it looks like an old floor register. If there is no air flow coming from it then my guess is that the previous owners at some point renovated and moved some walls around and rerouted the supply line elsewhere and instead of removing the old line just decided to leave it since it was inside of the wall. As for return air, usually it is done with one large duct but sometimes it is done with multiple if there is more than one floor or the furnace/heat pump supplies multiple units in a house but it is not usually in every room. As long as the air can transfer out of the rooms (ie. most doors have a gap underneath them) and make it to the return air duct that’s fine.