You can search for things like “anonymity” and “anonymous” to find a series of previous conversations on this topic.
In short, since we’re dealing with redistributable, open source software, there is no way we can guarantee that a site owner can’t read your “anonymously” posted content.
I think a better way to achieve truly anonymous feedback with Discourse still in the mix would be to set up an anonymous form (using Google Forms, Airtable, a WP plugin…) that mails in feedback on behalf of users.