That's no longer true as of around 5 years ago. They'll show ads on any video they like.
https://support.google.com/youtube/answer/2475463?hl=en "YouTube may also place ads on videos in channels not in the YouTube Partner Program."
If you become eligible and join the YouTube partner program I believe there may be a toggle then, but for non partners it's completely up to YouTube.
And the forced sign-in of course never was controllable by creators.