An unlisted post is hidden from latest and similar topic lists. But everyone who has the link can read it. And it also causes notifications. So when you mention a user or they watch the category, they receive a notification, which is a link to the topic. Then the user can read the topic. See also the documentation about unlisted topics.
You can create a category, which users cannot see (or use the default staff category), post your topic there and move the topic to another category to publish it.
Or you can use the shared draft feature to draft and publish your topic.