Threads has a list of all their blocked servers now, https://www.threads.net/moderated_servers

They also have a way to appeal your banned server, though it looks like it requires an Instagram account to initiate. Still, I think more ActivityPub software should have some method of appealing server blocks- I know I've blocked servers for spam that I just haven't had the time to go back and see if they've dealt with their spam problem.

Side Note: There's some terrible domains on that blocklist and I wish there were a better way for me, a single user server, to keep an up-to-date blocklist without spending 24/7 browsing the federated timeline