To the best of our knowledge, Meta is not hiding posts that contain the term "Pixelfed", here is a series of posts that were made to explore this issue:
https://www.threads.net/@andynico/post/C4upg5NKOs-
https://www.threads.net/@mogoh_viol/post/C4xsvpKMZb8
https://www.threads.net/@laurenshof/post/C4xse1Stvkj
It may be true that specific accounts are private, muted, or CW'ed for reasons we don't know.
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Please note, a server-wide domain block only stops content coming in, not going out.
Personal blocks work differently than server blocks.
You can block your public content from being retrieved by Threads users by personally blocking the domain.
Search for any threads.net user and block the entire domain using the three dot menu next to their profile to "block domain threads.net"
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You can also block one or more domains by uploading a csv list of them to your account, as described here. Clumsy, but useful when you don't have a handy post from the offending domain. https://medium.com/@geofbard/how-to-block-server-domains-in-mastodon-899b24f8fb6e