It may be a simple weatherstripping that was added.
It is rare to completely block a trunk line. In fact I have seen trunk lines split open because they were blocked off. The home owner did not understand what he was doing when trying to convert a multi level home from 60-40 to 40-60 and split a large duct in the wall.
In another case the blocked both main trunks with the fan on and it split the duct from the Y to the furnace.
The few times I have done this is for garages or basements not normally conditioned so a more positive or complete seal is made.
To do this I have used weather stripping. I have used silicone, but have found metal riveted in place with a compliant rubber stays in place and seals better.
The quality of the seal and how well it is installed can seal close to 99% and self adhesive sponge rubber seals can do as good but don’t last long and can blow out with higher duct pressures.