Leave it for the housing, I guess.
One of the common thematic points in my posts is that growth is less important in this game than it was in civ5. In civ5, growth for the sake of growth only (example, farming a flat grass tile which yields only additional food) was an effective strategy. Production and other yields were important as well, but food was the power producing yield. In civ6 production and gold, with faith as an honorable mention, are the king yields that get you stuff that helps you win. Science and culture are nice as well since they unlock more powerful versions of stuff, but the ability to produce more or faster trumps faster growth, especially since certain baseline science and culture yields (from population, or bonuses from some resources, etc.) are sufficient. Speaking of baseline yields, there is more than enough food available to have your cities grow at an acceptable rate without trying to maximize growth. mines yield one more food from their respective terrains compared to civ5. Granaries, which are needed for efficient housing, give food, internal trade routes give food... food is abundant everywhere. It's much more important to get your builds done a few turns quicker than it is to grow a few turns quicker, especially since the critical mass for cities in wide empires is 8 or 10 for most strategies as opposed to 30 or more as it was in civ5. The there's also the quick boosts from chopping rainforests and marshes. Personally, I like to get all my cities (except the capital which gets bigger) to size 10 - since it's an even number, your not "wasting" a half of an amenity's worth of happiness (9 or 10 require 4 amenities but 11 requires an extra), it's the level that unlocks the fourth district, which I find to be a good level of flexibility (CH and victory condition district are two, either an IZ or a EC for coverage is the third, and the fourth district is something that boosts auxiliary yields of science or culture as desired. Also, I want one city that has all the districts that produce more hammers from internal trade routes, which coincidentally is unlocked at size 10) Also, size 10 is about the maximum for having cities about 5 tiles apart and still work predominantly the higher priority tiles in most cities.
But back to the farming... I'll try to get a farming triangle in most cities for feudalism, especially if they're on plains tiles as they get some production as well. But that's usually about it, I don't build a lot of farms. To try to crank out a little more when there's excess amenities, I'll use feudal farms to get an extra 2 or 3 pop, but more often I'd rather use it towards another city, or just enjoy the bonus production from excess amenities. I often have farms that are not worked just to provide extra housing, and they are needed for that reason.
