In Germany i do not know any bank that does not support [0]FinTS when using online banking with your account. I would consider that a documented API for externa

In Germany i do not know any bank that does not support [0]FinTS when using online banking with your account. I would consider that a documented API for external use. However, you asked for american banks and i haven't seen options to get a sandboxed variant albeit i am sure these exist. Another option would be to open accounts at some Banks.... [0] https://en.wikipedia.org/wiki/FinTS https://en.wikipedia.org/wiki/FinTS