Entities that go near your data:
The other part of it is that I may have to look at tweets from your home timeline to debug an issue, though since I use this app I don't expect to have any problems manifest that I wouldn't have already seen for my own account.
This app only does reads from your twitter account. The "login with twitter" flow should indicate this.
Twitter account/status data is generally stored in plaintext in google cloud datastore. The tweets collected for each digest are held for up to 14 days at which point they are removed.
Emails containing your twitter timeline in plaintext are sent through sendgrid. All tracking pixels/click tracking settings are disabled for that sendgrid account.
Login with your twitter account, then uncheck "Yes, send me digest emails every week."
This is done from twitter itself. Go to Connected Apps configuration on Twitter