Manifest V3 was well on its way to being implemented before the aggressive youtube advertisement push. It was well known (to people who cared) it would kill uBlock back in 2019.

Here’s an article:

https://www.bleepingcomputer.com/news/software/google-begins-testing-extension-manifest-v3-in-chrome-canary/

Discussion on the uBlock subreddit: https://old.reddit.com/r/uBlockOrigin/comments/dunod2/google_begins_testing_extension_manifest_v3_which/