I think YouTube now puts ads on basically everything though... (I don't know for sure, I have had Premium for a little more than a year now, and before that uBlock Origin took care of things).
> If your channel covers certain taboo topics it can be "demonetized" which means no revenue from Google.
That almost never happens. Rather, individual videos are demonetized- or deemed unsuitable for all advertisers. In fairness, that can easily happen to an overwhelming majority of a channel’s videos, but it is important, IMO, to speak accurately about what is going on.
Source: I have been a full-time YouTuber for the last seven years. I publish content that often toes the line between suitable for advertisers and not.)
I believe they don't put ads on most demonetised videos, since they are typically demonetised because (YT's algorithms think that) advertisers don't want their ads associated with the content.