Content visible only on preview url?

Nope.

I think it’s possible. In your template file (e.g. single.php) you can check for the parameter “_ppp” used by public-post-preview.

Via $_GET[‘_ppp’] or https://codex.www.ads-software.com/Function_Reference/get_query_var#Custom_Query_Vars

example:

if (isset($_GET['_ppp'])) {
    echo 'This is just a preview.';
}

Awesome. That solution would be useful for adding a sidebar or widget via a template tag in place of the echo to allow for more easily customizable per-post messages. Thank you.