Google has quietly introduced a new automated conversion action tied to advertisers’ YouTube channels.
Alongside tracking YouTube subscribers as a conversion goal, the new action called YouTube follow-on views measures users who watch a channel’s organic videos after seeing an ad for that channel.

It’s automatically created when the YouTube channel is linked to Google Ads and is set as a secondary conversion by default. It can be also chosen as a primary conversion for Demand Gen campaigns with YouTube engagement objective.
Google has also published an official help page about this new conversion action, spotted by Hana Kobzová.
UPDATE: Follow-On Views aren’t actually new. They’re simply a rebrand of what was previously called Earned Views.