I have created a free, comprehensive code solution for creating "draft" portfolio items:
The reason I felt the need to create this code solution is because a client came to me needing this exact functionality. The existing accepted solution in this thread only takes into account hiding the post on the main portfolio page, but people can still navigate to it using the pagination from another post. Also, because it requires changing the URL slug, if you want to set an older post to draft that has already been indexed, then people will get a 404 because the URL has changed when they click that link.
My code takes all of this into account. Pagination of draft items are hidden, and the URL slug doesn't need to be changed. You simply change the post title from Post Title to (Draft) Post Title. If someone tries to navigate directly to the Draft item using its URL, they will be redirected to the main portfolio page.
I think I have truly thought of everything with this plugin and it should be a good solution until Squarespace wakes up and finally implements the feature themselves. I have a video on the post that walks through the entire functionality if you are curious about how to use it.
I would love to hear people's feedback! Hopefully this saves some people from needing to leave the platform.