I don't think it's 'often' and it's only in cases where there is a real and credible threat of harm to the witness.
While that might make it harder to go after organized crime, the US believes it is important that, when you are accused of doing something, you get to confront your accuser.
There's a really interesting law review article about this from 2020 which gets into the idea that, through misconduct, a defendant can waive their right to confront the witness at trial. Here's a link: https://digital.sandiego.edu/sdlr/vol39/iss4/3/
That makes sense and generally tracks, but I guess I'm not sure I feel like it's 100% an issue, especially in a civil trial like this. Plus, in this case, this isn't the accuser/witness, this is the defense, right? Reddit is roughly filing an amicus defending the case, so it makes even more sense that it would be ok to offer your experience and opinion on a civil issue without having to publicly say who you were, assuming your relevance and authority on the subject could be verified.
Most constitutional rights in the US aren't limitless and are subject to various balancing tests, of which this seems like a reasonable and relevant one.
I was responding to your statement that you thought that this was a common process in criminal trials, which is why the comment is only about criminal.
I don't entirely disagree with you re civil cases like this one, though I believe under normal circumstances it is preferable to have de-anonymized witnesses.
While that might make it harder to go after organized crime, the US believes it is important that, when you are accused of doing something, you get to confront your accuser.
There's a really interesting law review article about this from 2020 which gets into the idea that, through misconduct, a defendant can waive their right to confront the witness at trial. Here's a link: https://digital.sandiego.edu/sdlr/vol39/iss4/3/