She's had it a long time... It could be arthritis. And gout is a form of arthritis.
https://www.backyardchickens.com/t/915762/gout-in-chickens
This looked sort of similar but more in the legs.
https://www.backyardchickens.com/t/912300/necropsy-visceral-gout
And this one is about avian gout...