VG24/7 reports that an anonymous Rockstar employee's LinkedIn profile momentarily listed PC alongside PS4 and Xbox One as platforms RDR 2 is being developed on. After some digging, we tracked down the account ourselves—but it appears mention of PC has since been removed.
Here's a clipping of VG24/7's screen, followed by our own:
This is far from official confirmation, of course, but it's a compelling clue that we may yet see Red Dead Redemption 2 on PC. And almost all Rockstar Games thus far have ended up on PC. We've reached out to Rockstar for comment.