Hidden pages
Tick Hidden on a page and the menus stop offering it, while it stays at its address. Anyone with the link can still read it.
Off the menus is not taken down
Writing a page and advertising it in the menus are two different decisions. The page editor has a Hidden box. Tick it and no menu offers the page, but a visitor reading its address sees it as normal.
A hidden page also stays out of the site's sitemap, so search engines are never pointed at it. Finding the page means having its address.
What organisers use it for
A directions page meant only for the people who have its link. A page that is written but not ready to sit in the menus. A handout address you print on a poster. The page stays readable for those who have the address, and stays out of the way for everyone else.
Hidden pages work the same on managed hosting and on a self-hosted copy, like every other feature.
Where the idea came from
Chalkie's Dogs suggested hidden pages, and they now ship on every hosted and self-hosted site. The credits page tells that story. Pages themselves are part of your public website, and the customising guide collects the other ways to make the site yours.
The spec behind it
The feature's behaviour is published as a product spec, in the same words the tests use: keeping a page off the menus. The source repository holds the rest.