I agree, it looks like a feather cyst.Thinking ingrown feather that's became infected. So indeed a feather cyst.
https://www.backyardchickens.com/threads/feather-cyst-treatment.1571593/post-26686114
https://www.backyardchickens.com/threads/found-this-growth-what-is-it.1602147/post-27272870
https://www.backyardchickens.com/threads/found-this-growth-what-is-it.1602147/post-27273030