Maybe Wolfram already effectively killed this project. It was actively developed for about 5 years (see https://github.com/mathics/Mathics/graphs/contributors), then Wolfram Inc. hired the lead developer 2 years ago, and basically all worked stopped...
The lead developer up until 2 years ago, Angus Griffith's LinkedIn says he's at Google. [0]
In April 2016, Angus replied to a comment of mine "Mathics is still maintained. There are currently two of us working on it. The last release was v0.9 in March this year." [1].
The original creator Jan Pöschko's GitHub profile states "JavaScripter making the notebook interface of wolframcloud.com" [2]. His bulk contributions to Mathics stopped in 2012, but he still makes sporadic contributions to Mathics - see here he merged a pull request March 2019 [3] - suggests Wolfram tolerate the existence of Mathics because it's not a serious threat.
As the creator of Sage, what's your opinion of the legal risks of creating a Mathematica syntax compatible clone like Mathics in the light of Wolfram Research's fairly aggressive assertion of its IP "rights"?
Yes, that's roughly accurate. Jan Pöschko (the creator of Mathics) went to Wolfram in 2012 and I went to Google at the end of 2016. Since then it's been largely unmaintained as I don't have the time to work on it unfortunately
