[1] https://github.com/OpenRCT2/OpenRCT2/blob/develop/src/openrc...
That being said, I'm fairly certain it was not cleanroom reverse-engineered.
The not-so-legal status, then, is "Atari does not care".
nothing against this software, just pondering over this ... what would prevent a malicious software to make a similar claim, and if we always trust the software we download, wouldnt that reduce the amount of reasons to use anti virus software in the first place?
Semi-related note, AV tests of the last 2-3 years consistently show the Win10 built-in Windows Defender perform on-par or outperform other AV products on security, and outperform nearly all of them on performance.