RDR1 on Steam

That was the story but then they managed to release Nintendo Switch port last year so this Steam version was incoming

My understanding is that it was infected by Groinlings - a parasitic fully digital trickster lifeform, which makes it difficult and indeed dangerous to work on the source code, unless you wish to be snatched to the wretched Glitch Dimension.

From what I heard the most common culprit for missing source code is losing the physical media the backups are on while moving office. Usually they don't lose all backups but rather part of it and completing the missing parts from older versions can be tricky since documentation on game development tends to be a mess unlike other types of software development, and after years programmers won't remember how the heck they did things originally.
Sometimes companies have to opt for hiring someone to reverse engineer parts of a game just so they can remaster it, or reconstructing those parts from scratch.

It apparently happened to Icewind Dale 2, which is why it didn't get the EE treatment like all the other Infinity Engine games. And I was just saying what I had read. Wouldn't be the first time we've been lied to.