The App does help you find what data is shared where, but it is indeed based on the data that is shared over email. Which might not include everything, no. But at least it'll flag the account instead of being some, old account you forgot about.
I know a lot of companies tend to push you to their forms. Which can be helpful for identification, but you don't have to follow they process. They're legally obliged to honor requests via email. The app helps you to track them.
I know Github has its flaws, but using for convenience. Not sure how it impacts the App.