Yes.....root cellars need good ventilation and be able to vent out ethylene gases generated by maturing organic material.

Yes I could use it, or yes I'd have to built a venting system for it?

It should not be air-tight. If you look at designs for root cellars, probably readily available on the internet, you will see that they have some sort of venting for incoming air and a way to expel air. The reason being that ripening fruits and vegetables give off ethylene and ethylene in turn forces ripening in other produce nearby.

Were you intending to put this old fridge underground, or partially so? Stick it in an unheated garage or outbuilding? Are you planning to use just the shell, or is it operational? Your post didn't really give that much information.

If you are thinking of using a second, perhaps older but operational refrigerator as a place to store produce...sure it'll hold goods for while, but you are right that correct humidity could be a factor and have stuff like cabbage mold and some stuff other than root vegetables have X amount of life and is not suitable for long-term storage anyway. Also factor in the cost of running a second unit for shorter term storage. I have an actual natural cold room in one house I own, built into the ground under a concrete porch. The one I had in this house is no longer functional because my husband heated the cellar. I do, however, have some background in fridges for storage, because I grow floral products commercially and have used it to condition bulbs and hold back blooms so they open on schedule. The commercial holding refrigeration units for flowers and produce have venting systems designed in.

I'm sorry this isn't an easy answer because there are a lot of variables.

For ideas, I suggest Mike & Nancy Bubel's book, Root Cellaring. It covers a lot of options other than just building a traditional root cellar.

Carrots, beets, turnips and the large Asian radishes store perfectly in a fridge. I keep a winter's worth of carrots and radishes in the vegetable drawers, and some of the other two for weeks after I dig them sporadicaly through the winter..