How to unpublish a page and still keep a draft?

I have a holiday sale page that no longer applies as the holidays have past.

• I want to keep the page for future design reference.
• I do not want the page to be accessible from the Internet.

I tried:

• Setting the page status to Pending Review
• Setting the page status to Draft

Neither of these status changes seem to have any effect as the page is still visible at the URL. I am not even sure why these settings exist for pages.

I could password protect or set the page to private, but neither option unpublishes the page.

I could do a 301 redirect, but I would prefer not to use this method. (SEO impact of 301 redirects.)

Is there no other option?

When I logged out the URL generated a 404.

Correct.

Why can I see the page when logged in? Is this documented somewhere in the codex?

I think this is something that codex has but some parts are still not written, like this part for draft.
In the case of draft post your permalink will look like this:

?p=2898&preview=true

And it is not available when you are not logged out as you noted — 404 error page is what you will get.

The same error you will get also for the pending review status when you are not logged in, but the permalink will be the same as for the published post if you are logged in.