It's all very simple, considering Youtube has had a feature to keep videos out of their search indexes since, I don't know, forever...? It's effectively a form of voluntary shadowbanning, used by people who just want to embed videos somewhere else or, y'know, simply keep them somewhat private. YT could just say "right, anything we object to, gets removed from the search index." Two-minutes job. If you want to be more proactive, stop them from embedding too, so they can't be reposted elsewhere. The videos are then effectively neutered and only the already-nutcase will see them, limiting the virality.

I'd imagine most of this type of thing (prominent anti-vaccine personalities) is externally monetized via donations, as opposed to directly through YouTube.

> Aren't they already 'demonetised'...

That affects only content creator. YouTube, even if not directly profiting from ads, profits indirectly from you staying on the site and moving on to other 'monetized' videos eventually.

Demonetised videos can be loss leaders, pulling views to other monetised videos and having the user stay on the platform.