Out of context this probably sounds horrible but good is there in keeping rainforests around? Why not just cut them down and building farms? The production and food bonus from chopping rainforest is a good way to rush a city and the farm adjacency bonus outstrips the value of keeping the rainforest. The minor science bonus to a campus district is inconsequential. Rainforest do not even provide an appeal boost so if you are going for high appeal it is better to chop them down and replant a forest. In Civ V, rainforest were good for science and gold with trading posts once you put points into the Rationalism civic tree but in Civ VI there is no good reason to keep them. Occasionally I do get some AIs to whine about the destruction to the environment and to be "carbon friendly", I replant forests in the tundra but it doesn't really matter either way. Would be cool to see the return of global warming from Civ I so there is a shared threat to the world caused by industrialization and deforestation. However, right now there does not appear to be any benefit or deterrent that would keep me from cutting down all the rainforests.