Makes around $5k/month now (down from $7k/mo previously), fully passive income as I haven't worked on any new features in the app for the past 1.5 years or so.
It does one thing, that people need, and does it well, for a fair price. I assume it requires minimal maintenance, except to keep up with Twitter's API (honestly I don't know if this requires much work, I guess it depends on how much the API fluctuates).
Twitter is a perfect example: removing "some" of your tweets is purposefully made cumbersome for users so that they get discouraged to manage their tweets. Searching through your tweets by date range or by keyword, and a button to delete all results? "Nah, too complicated for us silicon valleyers" :)
One thing to remember with GDPR is that it is not a law that protects customers. It is a legal framework that specifies a set of requirements, which companies must abide to in order to do whatever they need/want with your data. Once you understand this, GDPR becomes much clearer :)