That is on the main branch. Behind the scenes [1] they're working a a huge rewrite to use the Bevy engine. A big effort that seems to be moving at a quite constant pace. It seems they're doing it quite rigourously: I've seen some issues in the bevy tracker where they check what is specific to their project, or where bevy can use some work.

[1]: https://github.com/nannou-org/nannou/tree/bevy-refactor