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Since airbending destroys grass with certain moves, such as airswipe, I think you should be able to toggle off airbenders ability to destroy grass in config.
It is most realistic for them to be able to swipe through the grass, and it comes in handy when fighting a water bender and you're in the middle of a grassy area; you can easily swipe at the grass to make a space they cannot use water manipulation from directly under your feet, allowing you to block shots.
I personally agree with this. It would be configurable. For whatever reason people would want it off then can, but otherwise, things wouldn't have to change for anyone else. Everyone could be happy!