Huh. I'm familiar with the Spring RTS FOSS project (https://springrts.com) which started as a reimplementation of TA, and of course Planetary Annihilation, but not "Beyond All Reason" - do you have an authoritative link?
The Spring RTS FOSS engine is an engine, but does not provide the game assets or game-specific code.
Recoil is a fork of the Spring engine (background: Spring made backward incompatible changes; Recoil forked to retain backward compatibility). Beyond All Reason uses the Recoil engine and supplies its own game code, shaders, and assets.
https://recoilengine.org/articles/choose-recoil/
https://recoilengine.org/ (list of games powered by Recoil)
https://github.com/beyond-all-reason
Source: been in the BAR Discord for about a year, have contributed tiny bits of server code to the project, and read a few pull request comments.
BAR is a great RTS
The official website is https://www.beyondallreason.info/
It's an open-source project that started as a fork of SpringRTS. To my eye it looks nearly like a clone of Supreme Commander.
I watched a few ranked 1v1 games on uThermal's YouTube channel (he's a former Starcraft 2 pro who mostly makes YouTube videos about Starcraft 2). Here's the playlist.
https://www.youtube.com/playlist?list=PL_USFDBbymGUwLPiopP2q...