Hello and Good morning Paul,
I believe search engines will index the page even if it is not in the main menu structure. I have run into this many times where a hidden/private form (that isnt visible to the general public) still gets filled out by persons trying to gain access to your system, or promote their viagra etc.
The only way that I know how to help control this on a page such as a private form is through the robots options.
This is also not a situation where the right people, who have been sent the private form link, should register with joomla prior to being able to access the private form. That would be cumbersome.
As a side note, Joomla also has the tools on the same page for metadata which helps with SEO. All of that seems to have not been implemented into page builder as of yet.
I hope we can have the same functions conveniently available in page builder pages.
Maybe these options could go in the left side where you have already implemented some publishing options?
Open Graph
Custom CSS
Publishing - (robots and meta added here ?)
Permissions
Thank you very much for your consideration on this.