Indexing within a certain time frame is not guaranteed.
Getting on top recent (again) with a major update devlog is not guaranteed.
And there is a possibility to get temporarily de-indexed, after each update.
Technically, a demo is not a game. You are not fussing about the release of your game, you are fussing about release of a demo. Actually, about early access to a "free" demo, as in constrast to all the non free demo versions... ;-)
You can't promote your game by existing on Itch. It will not work. There are too many games here. It does not matter if your non-game is indexed or not. You need to do promotion. And promotion will not care, if it is indexed or not.
I have doubts that the Itch crowd will "sign up". Why should they? Seriously. Why? If they want to be notified about your release, they can just follow your developer account and get notified about your releases. No need to give you personal data.