So providing it on itch.io as a sideloadable game seems moot. Would a downloadable demo be still considered as only promoting the full online game?
I've seen several projects that are precisely "demos" that have a link to Steam to buy the full game.
Personally, I don't like them, but from what I understand, they don't violate any rules and have been indexed by Itch, so I don't think there would be a problem on that front.