Since SAZ mentioned 10" deep, I'm guessing the bottom is that they are essentially making a soil tray and then raising that to get to a better working height.
DDF is right, another alternative would be just to build them 3' deep, but that would take literal tons of dirt to fill.
Yes, the concern with the soil tray idea is that the bottom will rot out after just a few seasons, depending on how thick it is. My raised beds are built out of 4x4" fence posts (cheap to buy) and they have lasted 10 seasons now. You could use something like that. Just hold the fence posts together with cross pieces underneath. That would allow you to leave some spaces between the posts for drainage. Whatever frame you are building will have to be very solid, dirt is heavy and using the fence posts makes it even heavier
If you are using plywood or thin boards they will only last a couple seasons without protection. You could put the plastic down to protect it, but then your issue is drainage. If you don't have good drainage, your soil tray will only be good for wetland/ swamp plants. I guess plastic with plenty of small holes in it. Your frame still needs to be very supportive. Thinner wood is lighter, but will warp more.
Here's some ideas for planter boxes:
It looks like some people are using plastic and some people are putting gravel in the bottom for drainage then landscape fabric, then the soil....