Is it possible to make a selected page of my site visible only to those who have the link and nobody else? (This functionality can be seen on Google Sites for example, so called "private link")?
Any page that is not linked from the other pages and homepage would be a private page, wouldn't it?
Does this mean it would not be indexed by Google as well?
I wouldn't be so sure. You call this a private page but the correct term is "unlisted". When Youtube tells you a page is unlisted, you can trust it because... they're Googles.
To ensure Google and most engines don't index your page, add these to the head section of your page (manually, after export)
<meta name="robots" content="noindex"> <meta name="googlebot" content="noindex">
This won't work with a Webflow hosted site as you can't edit each page head section and you sure don't want to add these meta to your whole site.