Java Edition 21w07a secretly added a feature that allows dedicated server owners to "censor" words on chat, books, etc using APIs if configured on the server.properties file. The information sent to the API includes the player's display name and the message to check. There is also a (currently dead?) method that apparently calls the API when a user joins or leaves the server. In this case the username is sent.
There is no indication of the system running (except by typing bad word in a book and using modded clients to check for filtered_pages NBT on the book.) - and even so, the user cannot know who owns the API and how they are processed.
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