I tested the current Github Version of Flatpress with PHP 7.3.1 and it looks really stable. The standard Functions of the Github Flatpress Version can be used without a displayed error and it seems that the new Flatpress Developer has done a great Job.
Not ready for production use, but just before
Edit: 16. Februar 2019
The current Github Version from 13. Februar 2019 is running stable with PHP 7.3.1. A few Tasks are open on the Github ToDo, but all looks really good
You can take a look on GitHub here:
https://github.com/f … tpressblog/flatpress