I have the situation with Chrome, tested with 87.0.4280.88, that in some cases Chrome will send sec-ch-ua and sec-ch-ua-mobile request headers, although chrome://flags/#enable-experimental-web-platform-features is set to "disabled".
For instance:
sec-ch-ua: "Google Chrome";v="87", " Not;A Brand";v="99", "Chromium";v="87"
sec-ch-ua-mobile: ?0
When this happens, the only way to get rid of these headers is to start Chrome with the parameter --disable-features=UserAgentClientHint.
Does someone have an explanation for this? As far as I understand, these features are still experimental and the specification in progress (https://wicg.github.io/ua-client-hints/).