When did Big Reactors stop getting maintained?
The last released version was released April of 2015. So....
However, I believe someone is doing an unofficial (or maybe official???) port of it. I seen it on Reddit, I believe.
I don't think anything official has been said about Big Reactors no longer being maintained, but it's a safe assumption that it's dead. Last release in April 2015, last time the dev was active on Twitter was May 2015, last commit on GitHub was over a year ago. And that's all I could find after a quick check.
Should be interesting to see if someone tries to continue it and to see if they decide to follow the roadmap that was posted.
Edit: I found a thing. It's the most up to date repository I could quickly find.