I didn’t put anything under mine with a wait and see attitude, but my deck is about 30” and I figured I could always crawl under later if I needed to. So far nothing much has been growing under there as there isn’t enough sunlight and around the edges I use roundup.
I’m not a big fan of the landscape fabric in flower beds but under a deck covered with a layer of gravel should work good. Let the water thru and stop anything from growing.