It's completely free to play (though they/we have recently signed a publishing deal and there will be a paid for campaign mode later) with a combination of GPL and MIT source code.

Free as a beer due to assets.

- All the code is GPLv2, MIT and other open licenses.

- Some assets are CC-BY-SA but there are also a quite a bunch proprietary ones

Built on the Recoil engine: https://recoilengine.org/

Which is a fork of the Spring RTS engine: https://springrts.com/

Fairly freedom-y by the looks of things:

https://github.com/beyond-all-reason

It was. Not anymore though.

Both, as I learned from 5 min browsing the site.