Just wanted to briefly chime in here. There was a period where development on Lexicon had slowed, however the project was not abandoned and it is still alive and well. We recently released Lexicon version 2 with support for push notifications.
Sorry to hear that focus on this project has shifted elsewhere for now. While our project is built with React Native, we are big fans of Flutter, and it would be great to see a Flutter-based project for building Discourse apps.
Regardless, thanks for your contributions to the ecosystem!