I don't understand why published notebook cannot be updated unless they are public. Is it possible to publish a new version of a published notebook?

We moved away from the concept of periodically publishing changes in favor of an always updated public notebook. So any changes to published notebooks can only happen once they have been made Public.

"Only you can edit your notebooks", does it mean "only you can save changes to your notebooks"? I can edit this notebook, but I cannot save changes.

Thanks. Clarified above.

Before the `publish` functionality could be used to incrementally update a notebook intended to be published, but only publish it once the work is ready. Seems like this is no longer possible for Free accounts, and its unclear if it is possible for Pro accounts. While a notebook that is already public is being edited there can be many cases where the notebook is broken or not ready, how can this be controlled now?

Please see the FAQ and discussion with some advice here: https://talk.observablehq.com/t/faq-october-2022-platform-change/7120

Is a public unlisted notebook findable by search engines?

No it is not. We block search engine crawlers from unlisted notebooks.

Attempting to navigate to the 15th version of this notebook ( https://observablehq.com/@observablehq/public-and-published-notebooks@15 ) results in the following error message: ***You either don’t have permission to view this notebook, or it doesn’t exist.***

Yes, that is because the first public version of this notebook is v50 (which is what you see in the history pane on the right of the notebook). The notebook was private before that and only became public at v50.

Just to confirm behavior: It looks like you can re-privatize a notebook after one has made it public. Let's say a notebook's cell(s) was/were imported at version X and the owner of the notebook makes it private at version X+5. Would the public be able to review the imported notebook/code/cell(s) for version X while it was public? Is it the case that once a version (not whole notebook) is public it will always be public?

Great question! Once you make a notebook private at version X, no versions are publicly visible anymore (well, mostly... see next paragraph). When you make version X+5 public again, all versions from version X+5 onwards will be publicly visible again. So the public visibility has a starting version. One exception to this is for versions that have been 'published' before (in the old model). All of those versions (individual version numbers) will still be visible to the public (and of course for imports and embeds that explicitly used those versions). However, they will not show up in the notebook history for the public (once the notebook is made public).