(is it possible to have a heavily planted aquarium that doesn't need water changes or filtration?)
Back when I started in the hobby, in the late 50s, these unfiltered tanks, were called "balanced aquarria".
It'd be a tank of maybe 3 mosquito fish, and 100 plants, or 4 neons and 200 vallisneria.
And back then, some aquarists thought water changes were bad.
Of corse back then, science was a far cry from what it is today, some people may have still believed the earth was flat, and disease came from evil spirits, and miasmas.