Content creators can choose to disable notifications on specific videos.

Creators sometimes do this if they publish more videos than usual, or of the video is different from their normal content.

Also when you subscribe you now also have to click the bell next to the subscribe button to get all notifications.

YouTube itself also has a built in subscription feed that lists all videos published by channels you subscribe to. It also lists the channels with an icon next to each if they have unwatched video.

I use Feedly for this exact thing! YouTube's recommendations were either distracting or just not relevant. Feedly embeds the video right in the reader so no going to YouTube even. And now I only get the videos I want to watch.

That's interesting, my subscriptions only update once or twice a week so I never felt that need.

How do you see it working? Create and play a playlist of currently unviewed videos?

Exactly, a "Make playlist" button on either a folder of YouTube channels, or on just a channel, that would make a custom playlist of all unviewed videos.