Published in the settings(options)
If you’re talking about GitHub Pro, where you can publish a Pages site on a private repo, yes, anyone with the link can view your site.
However, it is entirely up to you if you want your site available to search engines.
If you don’t want your site available to search engines, then add the noindex meta tag into the head of all your pages:
<meta name="robots" content="noindex">
And for GitHub Free, it is not possible to publish a Pages site on a private repo. All Pages sites must be on public repos.