You're technically right (I think). However, I believe if you witness a murder and know the murderer and the police asks you: "Do you know anything about X murder?" Then I think you're legally required to tell the truth here.
Not sure whether that extends to police questioning though.
It still exists in many jurisdictions, including the UK, the US (it is a federal crime under 18 U.S. Code § 2382, and also a state crime in most states), Australia, Canada, New Zealand and Ireland.
Related was the crime “misprision of felony”, which was failure to report a felony (historically treason was not classed as a felony, rather a separate more serious category of crime). Most common law jurisdictions have abolished it, in large part due to the abolition of the felony-misdemeanour distinction. However, in the US (which retains that distinction), it is a federal crime (18 U.S. Code § 4). However, apparently case law has narrowed that offence to require active concealment rather than merely passive failure to report (which was its original historical meaning)
Many of the jurisdictions which have abolished misprision of felony still have laws making it a crime not to report certain categories of crime, such as terrorism or child sexual abuse
Or you can just not respond, at least in the US. https://en.wikipedia.org/wiki/Self-incrimination
If you're the witness to a murder and you're subpoena'd to court and refuse to testify then you are committing contempt of court. There was a guy in Illinois who got 20 years (reduced to 6 on appeal) for refusing to testify in a murder.
https://illinoiscaselaw.com/clecourses/contempt-of-court-max...
Contempt of court usually has no boundaries on the punishment, nor any jury trials. A judge can just order you to be executed on the spot if you say, fall asleep in his courtroom. Sheriffs in Illinois have the same unbridled power over jail detainees.