I just want to add that I have seen this many times.
The furnaces came shipped with a horsehair filter in the bottom and IF the bottom was not used for RA, it was fairly common for the filter to be left there as was rather than replace it with a sheet metal bottom.
It doesn’t hurt a thing in there although it is not needed either since it is not doing anything. I assume that the installer(s) felt since the furnace is setting on a solid surface, no air can get in but IF any does, it will be filtered. (in the ones that I’ve seen like that)
Having said that, I can’t see any RA interning the furnace in the pictures plus the one picture looks like the RA grill is allowing air to enter under the furnace floor and up through the bottom of the furnace, like most typical bottom RA furnace applications.
Of course we all know that can’t be possible IF that is a piece of sheet metal on top of the filter.