The Elm Architecture is not that super novel. Actually it came from game development from the Components way of thinking. Take a look at this book. It's a classic for gamedev: - https://gameprogrammingpatterns.com/

The language itself seems interesting but from the outside looking in it appears a bit stagnant.

Is the community active at all in 2021 or is it just the result of having basically one person working on the language?

Elm seems stagnant because Evan is not so transparent and prefer work in a private branch and to release work in batches instead of having a release every 6 months, this makes people think it is dead. Elm is still being used in production by dozens of companies with huge codebases (100k~400kloc), and the community is very active, there are amazing projects like elm-pages, elm-review, elm-ui (from mdgriffith), elm-spa, elm-charts, lamdera and others.

check out the elm slack! Very active, but that's behind a signup wall so that's ahrd to see from the outside. https://elmlang.herokuapp.com

Most of the activity happens in the (public) Elm Slack community, so it's not indexed in search engines and hard to find in StackOverflow, but it's a really effective place to get advice and help with a problem in a more personal way

I would say it is quite active, although still small.

I think there is a slow shift toward community built tools such as lambdera. The language itself is pretty stable (and good, IMHO).

Most of the news are available on the discourse discourse.elm-lang.org

The Nu game engine might interest you. There's also FuncUI for app dev which is F# + MVU running on Avalonia.

https://github.com/bryanedds/Nu

https://github.com/fsprojects/Avalonia.FuncUI