Having this same issue. Basically can't add a draft project, or hide a project without deleting it?! Any new project I add is immediately available to view (even if it's still a draft) unless I hide my entire portfolio. Can't believe this still hasn't been resolved. It's a major oversite for a template that's supposedly made for portfolios. Disappointed but not surprised bc I've found the UX to be bad overall on Squarespace.
Also tried some of the custom coding suggestions, but didn't have any luck.