The Witness will be released to iOS and PC after its PS4 release.
Even though The Witness is a PS4-exclusive title for the time being, the exclusive nature does not include releases to iOS and PC. The creator, Jonathan Blow, further discussed the choice he made between Sony and Microsoft in an interview with Kotaku:
“The Wii U still has pretty low system specs, so it came down to a choice between the next Microsoft console or the next Sony console. There were people at Sony who really liked the game and were keeping in touch with us about it, and so we naturally started going to their PS4 developer events, got a dev kit, and started playing with it.
“I don’t have good communication with anyone at Microsoft right now, and haven’t been disclosed on their next console, but all our technical people like the PS4 specs a lot more than the leaked Durango specs, and we like the positioning of the PS4 (it’s about games) a lot more than what we perceive Microsoft’s positioning is going to be.”