There are a lot of ways to announce adding a new platform to your game’s release lineup. You can do so at a major convention, put out a new ad, send out a big press release, add it to someone’s profile on LinkedIn… the last one is admittedly not traditional, but it’s the biggest piece of evidence that Red Dead Redemption 2 is indeed heading for the PC.
A developer whose identity remains unstated updated his LinkedIn profile to reflect his work on RDR2 for various platforms, including the PC. He has since removed the reference from his profile since it seems to be an obvious mistake, but the Internet never forgets these things. Again, it still doesn’t quite have the appreciable tang of having an actual announcement about what platforms the game will be playable on, but it seems like pretty strong evidence in favor of a desktop release. We’ll probably know for sure as October gets closer.