Any thread that has a public share link can be shared publicly. Anyone who has access to the link will be able to view the thread and discussion, and sign in or sign up, to join it.
If you're just getting started on Threads, sharing a thread publicly can be a great way to get your team involved in Threads.
Note: new organizations will have public share links automatically enabled. Only organization administrators change this setting. This can be done under the Security Settings in the Organization Settings.
Learn more about sharing threads publicly.