It's both A/B tested and a very frequent cat and mouse game with ad blockers. Youtube was pushing out updates daily, and uBlock Origin was responding in kind within hours (Sometimes you would need to manually update your extension though). The last few days Youtube hasn't put out any updates[1]. I believe the last change was 3 days ago.

[1] The current youtube ad notice version can be found at the bottom of this link. No dates, but it's useful to compare if you knew what the last uBlock Origin filter version you were aware of. https://pastefy.app/G1Txv5su/raw