And Chess source code is also still published: https://github.com/apple-oss-distributions/Chess (Despite what the readme says, the code seems to be for the version shipping on Mac OS 26).

The engine that Chess.app uses is GPLv2

	▲	microtherion 17 hours ago \| parent [-]
		Yes, but the engine is running in a separate process, and communicates over a generic engine protocol, so that is no reason that would force open sourcing the GUI.