There's a KnowYourMeme [0] post with additional details and context. Most interesting to me is finding out that there' s a word filer / transformer, so SMH becomes BAKA and TBH becomes DESU, as two examples.
The term “weeaboo” as a term for western anime fans only came about because it was what the word “wapanese” filtered to. It was originally a nonsense work made up in a Perry Bible Fellowship comic.
From what I heard, it was because they were tired of people posting "tbh fam". This does result in people instead posting "tbdesu" in being aware of the filter.
A note that the filter for "doublechan" was never updated to include its current name, nor the place where this current attack originated was ever filtered, afaik.
[0] https://knowyourmeme.com/memes/events/april-2025-4chan-hack