YouTube has a “Don’t recommend this channel” option. Which, as far as I can tell, does actually get them to stop recommending the channel on the main recommended feed. If you’re subscribed they’ll still show up in your subscriptions and will still show up in search if you look for them (to the extent that anything relevant shows up in search).
The option isn’t prominent, it’s in the “3-dot” menu next to a video on the recommended feed and I’ve been unable to find a way to view or manage the list of blocked channels, but it’s there.
Edit: a word - “able” to “unable”
He didn’t use encrypted everything. He had a public telegram group chat in which he stored a lot of his material. Which, as many people in the comments on the article pointed out, is not encrypted, but is presented by telegram as if it is. That’s likely how they caught him.