My current understanding is that the demo is going to get one more content update before the dev switches focus to releasing WIP builds that are exclusive to supporters on subscribestar.
I don't think Finchy has decided yet if there will be more free content updates or not. And if there will be updates I don't think it's been decided how those updates will work, whether they will lag behind the paid builds or what.
All that being said, there will always be some content that is exclusive to the substar because the rift pack DLC is on substar with exclusive content that can't be on the itch page either because of copyright or content guidelines.