Unfortunately due to the sidescrolling nature of the genre widescreen / ultrawide would break the game.
Viewing post in Pure Onyx - Alpha Release October 2024 comments
Yeah we supported it for a while but it became impractical once we started adding lots of events that need to sync with parallax driven background art :(. It would kind of need to be developed for the widest format from the get-go and adjusted for 16:9, but since that's by far the most common aspect ratio it doesn't make a lot of sense.