I agree, the article isn't clear and even Youtube's announcement isn't particularly clear either.
Normal annotations were discontinued a while ago so it's not that.
I've done a bit of digging and dicovered that what they are discussing is the "featured content" option (ie the little pop up thumbnail of a featured video that will appear across all videos on a channel) ->
https://www.youtube.com/featured_content On that page, there is the reminder message that they're discontinuing the option on 14th December so it's definitely that. - I'm actually quite disappointed - It is / was a great feature to promote a channel's newest upload. I find it hard to believe that YT are stopping it due to low CTR - People must have been abusing it. Shame.