I'd for sure go over it carefully with a metal detector first. And not use my best, newly sharpened planer blades. Looks pretty far gone to me.

check very carefully for nails. personally , i think you'll have a large pile of sawdust and no solid wood at the end.

They are 50% composted already.
Casey

A metal detector won't be good enough. Chances are those cracks are filled with dirt, sand, grit and stones which will chew up your planer knives unless they've been neatly stacked, stickered and covered. That's unlikely otherwise the boards wouldn't have decayed like that.

Saw it in half with a hand saw or chain saw to see what the inside looks like.

I wouldn’t risk a power tool on it - it's way past weathered into rotten.