Interesting - thanks. It looks like at least the Android and iOS apps have been updated in the last few days - so it is under active development:
https://github.com/deltachat/deltachat-android https://github.com/deltachat/deltachat-ios
Doesn't work with all email providers though, from their FAQ:
Proton mail isn't supported (I'm guessing because of the way Proton encrypts your email at rest?).