At this point, we're pretty sure we'll be seeing Red Dead Redemption 2 on PC, as the evidence continues to mount up. The latest hint was spotted by JakoMako51, who shared his find on Twitter. In the Rockstar Social Club source code, there's a single mention of the port we've been waiting for: "RDR2_PC_Accomplishments".
Now, 'PC' could refer to other things, like player character, but the context suggests this refers to the platform and achievements, as it sits right next to "RDR2_PS4_Trophies". It seems highly unlikely it could be anything else.
You can take a look yourself by heading over to the Social Club and browsing the source code. If you're using Chrome, you can open the dev tools just by hitting F12, then you can select 'Sources'. I've also taken a screenshot for posterity.
I reached out to Rockstar just in case they fancied announcing Red Dead Redemption 2's PC port on July 4 and will update this story if I get a response.