Yes, it is possible for guests to post comments and create new topics without registering. However, this depends on how the posting and comment settings are configured.
For new topics, you can configure the Permitted Poster Roles in WP Discussion Board so that posting is not restricted to specific registered user roles.
For comments, please also make sure that Users must be registered and logged in to comment is disabled under WordPress > Settings > Discussion.
With these settings configured accordingly, visitors can create new topics and post comments without registering an account. But just a reminder, allowing guest users to post without registration can make the forum more prone to spam and unwanted submissions, since anyone can submit content without having an account.
We recommend enabling this option only if guest posting is necessary for your use case and considering additional spam protection measures to help prevent unwanted posts and comments.
Please let us know if you have any other questions. We’ll be happy to assist.