Depends if you're using Lemmy through a browser or app. Browser apparently doesn't have this feature yet, which I just learned because I only use app for Lemmy, but I found this: https://github.com/Zetaphor/lemmy-keyword-filter/ and if you're using an app it can be found in settings and is under "Posts/Comments/Filters/Blocks" depending on what app you're using.
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