When I run yarn in a directory that includes a package.json file with a "private": "true", field, this line is silently removed. This behavior is in yarn 3.1.0, but not in npm 8.5.5, for example.
Running yarn seems to be type sensitive!
"private": "true", -> {a string} is removed."private": true, -> {a boolean} is untouched, and should work as intended.