At the moment this is not implemented yet, so this is a feature request, not a bug report.

Ah I see, apologies. As the Spaces user guide lists one of the possible use cases for Spaces as:

An organization might make a space for a project in order to satisfy a contractual obligation to limit access, even internally.

I'd presumed not being able to limit a project's visibility using a Space was a bug.

"project" there just the noun, not "Project-object", but we could probably pick a different word to avoid ambiguity.

Got you. Hope to perhaps see this feature implemented at some point in the future then, as it would be a big help to us on our client projects.

It might also be helpful to update the "Limitations and Caveats" section of the Spaces User Guide to ensure others aren't confused regards this limitation.

As it also says in this section:

As with normal policies, users may be able to determine that a #yolo project exists, even if they can't see it: they can try to create a project using the #yolo hashtag, and will receive an error if it is a duplicate.

It clearly makes it sound like restricting the visibility of Projects is included in Spaces functionality.