Sorry, I never got notified of this reply.
For the sake of anybody checking this thread, the answer is no. The game's content, be it on Steam or Itch, does not differ at all.
I believe the art was done in 4 weeks, though some may have been polished or added post-release (since it's up to version 1.11).
The writing was more like 1 week, IIRC. Not sure how long the translation took.
Music was creative commons (at least originally, might have been replaced), so who knows when that was done or how long it took.
So yeah, mostly done within 4 weeks. Sendo worked tirelessly on this one, and I think it really shows.
No, if you purchase Sickness on Itch, it does not come with a Steam key.
However, Sickness is currently on sale on Steam for the same amount as here, so if you'd rather the Steam version, you can still buy it there.
glad you enjoyed the demo. I hope the full game lives up to your expectations. Sickness is pretty long and eventful, so it branches out a lot more after the demo.
Sickness is actually on sale on Steam at the moment (35% off), and will be on sale again in a couple of months (50% off) both here and on Steam when the next update is released. There's no clear ETA for the next update, however, so I can't be more specific on the timeframe.