It'd take a whistleblower for us to know about it. Companies have been repeatedly caught filtering politically sensitive topics inappropriately (yahoo comes to mind https://archive.thinkprogress.org/yahoo-appears-to-be-censor...) but it's usually explained as an "error" and I suspect that in many cases it genuinely is, it's just impossible for us to tell the difference between "caught them red handed censoring" and "caught them red handed being bad at their job"