itch.io

From the Discord server:

No worries! The private game feature only just recently got released on the 4th March update, so it's brand new!

You can mark specific games as private and have it fully hidden on your profile: https://help.steampowered.com/en/faqs/view/1150-C06F-4D62-4966

Just as an info so it's also seen here, they did update in Kickstarter and on Twitter occasionally. The latest update is that the game is going to be released on the 12th March. So exactly one week as of this comment.