Hypothetically, if you are a witness or whistleblower who's adversary was a very big corporation (such as boeing), what measures could you take to prevent yourself from being "suicided"?
Only use encrypted communication, use a phone running GraphineOS, and your computer should be running a secure OS like QubesOS (in addition to Tor and/or MullvadVPN). Basically you need maximum security and maximum privacy, a Thinkpad running Libreboot is the best laptop and a Pixel will be the best phone.
Good suggestions, however, I don't think even pigeon carriers will stop yourself from mysteriously dying of suicide with 2 shots in the back of your head.