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.