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 food for thought. And almost all Rockstar Games thus far have ended up on PC. We've reached out to Rockstar for comment.