UPDATE So as new info comes out, I’ll be posting it here. It seems as if this Rollout Has Several Parts.

Part 1

You get a popup message over top of your video, blocking the screen:

• This is the first sign. If you see this popup AND are logged into a YouTube account, your account has been selected.
• At this stage you can likely close or block these messages with an adblocker.

Part 2

This message will change, indicating that you have 3 remaining videos to watch without ads.

Will insert photo once one has been found

• At this stage your adblocker will imminently stop working in 3 videos time.
• Personally using Firefox + uBlock Origin and tweaking filters and updates does not even fix it.

Part 3

None of the video loads now, everything looks blank.

• At this stage you must tred new ground to avoid ads. I have posted methods in the comments. If you want to bypass this end page, read down there.

End of Update

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YouTube has started rolling out anti-adblock to users inside the United States, which means that they are preparing to roll this out to the entire country. Personally, I have been blocked already. I want to gauge how common this occurrence is.