When the furnace was originally installed, the first floor of my house had four cold air returns, two each on the long walls. When the kitchen was renovated in 2015 (before I purchased the house) some ill-conceived changes to the HVAC were made, including the removal of the grate for the cold air return in the kitchen, meaning the first floor is now served by only three (despite still having four supplies).
There's now a roughly 10' section of cold air return ductwork that is essentially a dead-end. Will this cause any issues with the functioning of the system? Should I block where this connects to the main return trunk? One of the challenges with the system is that upstairs there are only two supplies and one return, and in the summer it takes a long time for the second floor to cool off.
In the image below, the blue lines show current/active cold air returns, going back to the furnace (blue square). Red is the one that is abandoned in place.