Plain suet is just fat. It's usually fat from beef cattle. The only protein content in it will be from whatever you put in as additional ingredients. Fat won't help give them protein, but it will provide extra calories they can burn to stay warm in the winter. Peanut butter and sunflower seeds added to it will have some usable protein in it. I'm sure they enjoy it and it would give them something to peck at.
Even though the feather eating is a habit now, I would still look at supplying them some additional protein and see if that helps. Free range chickens should have plenty of foraging to do, to keep themselves busy, so I'd be concerned that their diet is still lacking in some way. I'd also add some sunflower seeds to their diet, for the methionine. Out of curiosity, what brands of feed have you been feeding them? What else have you been feeding them? Do you leave food available 24/7?
Even though the feather eating is a habit now, I would still look at supplying them some additional protein and see if that helps. Free range chickens should have plenty of foraging to do, to keep themselves busy, so I'd be concerned that their diet is still lacking in some way. I'd also add some sunflower seeds to their diet, for the methionine. Out of curiosity, what brands of feed have you been feeding them? What else have you been feeding them? Do you leave food available 24/7?