I've never got one of those anti-adblocker messages on YouTube and I just use a vanilla browser with no addons and I get zero ads. The Channel 4 website won't let me watch anything without disabling the adblocker, so it's not like my browser is in some super stealth mode where servers can't tell I'm blocking ads. I'm not complaining though.
Nope, not using Firefox on mobile. (I do on Linux mint though, and even with ublock I still got YouTube ads last time I was able to actually connect to the Internet.)
Google sells youtube to twitch, suddenly a multi-trillion dollar company ...
Yeah, adblockers are a solved issue and only still work because google knows anyone using them will leave before watching dozens of ads per video, and those numbers are high enough to be a real threat.
It still exists, but was hidden for some reason. You can find it by going to the support tab and clicking the "more" button under troubleshooting information. It'll be at the far right of the button list on the resulting screen.