Do you mean you're trying to access Threads' ActivityPub endpoint(s)?
I'm pretty certain that you have to use signed requests, and bear in mind that not all users have federation enabled.
I just confirmed that by using the fedify
CLI to test. Running fedify lookup [email protected]
fails, but with fedify lookup -a [email protected]
(which spins up a local server and performs authenticated requests) I get back JSON from the WebFinger endpoint.
You'll probably get some folks able to share their experiences integrating / testing with Threads federation in some of the Fediverse Developers related Matrix rooms.