YouTube is going to stop the video annotation feature from 15 January next year. YouTube introduced annotation feature in June 2008 were a Creator can add text, links, videos to the existing videos.
The reason behind this is Increase in the number of users browsing YouTube from mobile devices. According to YouTube, 70% of its traffic is coming from mobile devices and since annotations only work on YouTube’s desktop site so it is reaching very less amount of peoples.
Annotation feature was disabled in May 2017, and from next year they will stop showing existing annotations on the videos.
YouTube suggests that you use tools like Cards or End Screens to feature your videos on the existing ones as both these features work well in the mobile version of YouTube.