Simple answer...Chewing = pleasure. Pleasure = reinforcement of behavior. Consistent reinforcement = habitual behavior.
It's unlikely that it's a nutritional deficit or pica. More likely boredom or frustration/separation anxiety. And note that this is probably a behavior that doesn't last long, but provides a momentary break from boredom or eases anxiety. If she chewed all day long, you wouldn't have a deck.
I'll bet if you had a nanny cam set up, you would find a pattern in this behavior. If she is frustrated or worried at your leaving for the day, it may occur in the moments right after you leave. If she just gets bored on occasion, it may occur in brief episodes during the day. In the first case (frustration/separation) providing something appropriate for her to chew or an activity (interactive toy) right before you leave, may help. Instead of just leaving toys, bones, etc. in the yard, make a point of tossing something out for her as part of your "getting ready to leave" routine. She has to be aware of it. Don't expect her to make the correct choice between going to look for her toy in the yard or chewing the deck.
When you're home, you need to supervise her constantly when she's outside or has access to those inappropriate objects, and then distract her or wear her out. Since she's been at this for 3 years, it won't be easy or quick, but it's do-able. You will really have to work at this. You can't just turn her out and then go about your routine. You have to watch. The only way to break a habit is to eliminate the behavior entirely, once and for all. If she gets to chew wood on occasion, she is just using variable reinforcement which cements the bad behavior faster. However, don't wait until you see her chewing and then provide a substitute. You may end up teaching her that chewing produces something better, even if it's just your attention. If possible, give her the chew toy, bone, etc. right before the bad behavior occurs. That's why you need to watch her and try to learn when that behavior is about to occur.
Appropriate chew toys/treats/activities have to be more satisfying than chewing the wood. You could try stuffed Kongs, large raw bones (never cooked and assuming no resource guarding in your crew), interactive toys, a sandbox to dig for treats in (may substitute one bad behavior for another...inappropriate digging for inappropriate chewing). Anything that is more fun than chewing on wood. You will have to rotate toys or chewies regularly to keep her interested.
Activities that wear her out and provide mental stimulus, like agility, long walks, swimming, doggie day care, etc., will help some. I see that she gets some of this now, but you may have to add more and/or more often. Try to make these activities unpredictable. If she senses that she's going to the park once a week or taking a walk right before you leave or come home, it won't help break the chewing habit. If the activities occur more often and/or at varying intervals, it's a whole lot more interesting for her.
If possible, cover or barricade her favorite chew targets (inside and out) for now and work on redirecting her behavior. Do not leave her with anyone who isn't prepared to supervise and work to correct this behavior.
And a muzzle only prevents the chewing while it's on. It never addresses the cause and may even make her become despondent and disabled by making her feel vulnerable and unable to communicate with other animals. I wouldn't do it. Never muzzle when you are not there and never muzzle her and turn her loose with other animals. They will take advantage of her disability, people will wonder if she is vicious, and she may become entangled with an object or other animal. There are numerous reasons not to muzzle unless a dog is a dangerous, unpredictable biter. Better off using a crate or restricting access to the doggie door when you're gone.
