[0]: https://www.eurogamer.net/skyrim-dlc-timed-xbox-360-exclusiv...
[1]: https://www.eurogamer.net/after-five-years-of-xbox-exclusivi...
[2]: https://arstechnica.com/gaming/2007/06/gta-iv-downloadable-c...
This sort of behaviour was well established in the industry.
All of this is shitty, but its like two pigs arguing who is more covered in mud.
Bungie used to make games for macOS. The predecessor Marathon Trilogy was originally released on macOS (and only Marathon 2 was ported to Windows). Halo was supposed to be originally released on macOS too, and then Microsoft bought the studio and turned it into an Xbox game, leaving all of the existing macOS userbase out in the cold.
Their purchase of Bungie also screwed up the IP rights to Oni. Wikipedia says that when Microsoft acquired Bungie, negotiations with Take-Two Interactive (which owned a share of Bungie) left Take-Two with the rights to Oni, and the sequel was eventually canceled. I don't have a citation on this but from what I understand, nobody is willing to touch Oni again (not even for an HD remake) because something about the IP rights is so messy that nobody wants to (or maybe nobody is even able to) untangle it. Which really sucks because it was a great game.
I've personally been involved in one business discussion involving folks from Apple about platform exclusivity for an app.
The app had been previously featured in the app store, and had no feature changes when the review process screetcyed to a halt. I'm hoping someone on HN remembers the details, but I'll try to Google and update this comment if I find the article. In the mean time, take it wirh a pinch of salt as I may be misremembering.
Of course modding difficulty doesn't translate directly to how well Proton will run it, but I fear it will make it difficult for it to be optimized.