What are some of the chess databases in the public domain that can be used for commercial purpose? What chess databases allow commercial use without having to reveal source code as part of their license? I see a few chess databases online, even the huge databases on lichess. I wonder which of these can be used as part of a commercial project without having to worry about licenses and unfair usage.
I don´t think chess games are copyrightable, as they´re just a collection of facts, so games of chess are not considered by the courts to be anyone´s property.  I would say that you are free to comercially reproduce a game, as long it´s just that, the moves played in that game.
To put in perspective, if you took a screenshot of the result of a videogame, for example Civilization, or if you recorded a song in Guitar Hero, then that would be the intellectual property of the creating videogame company.  Following that pattern, any PGN would be the legal property of whoever invented chess, but I believe the copyright may have expired by now.