Are you asking about elm-ui, or about Elm the language?

Elm libraries (like elm-ui) are generally very stable (thanks to the language guarantees), so the fact that it was last updated at May 2017 isn't necessarily bad. In fact most of the libraries, when they're published, they're done. (Sans occasional bugfix)

I don't know about this particular one though, it seems the author is focused on non-Elm things now: https://www.mint-lang.com/

Elm the language is active, alive, used in production, the community grows, it seems that the alpha release of v0.19 will drop any moment... Things are looking pretty good!

(PS: The Elm slack - http://elmlang.herokuapp.com/ - is probably better place to ask Elm-using-people stuff :) )