My first thought was that it had escaped from a predator but it has no sign of injury. I think it must have some disease or infection to affect its feathers and I guess it must be fairly long standing, given the feather deformity and discolouration. I didn’t get far finding the answer online. If you google “damaged tail feathers” all you find is loads of stuff from budgie and cockatiel breeders!
I feel for this bird because this is how I look most of the time. If you have any idea what it is please leave a comment.