YouTube has announced that the feature that allowed creators to add credits to your videos is being discontinued on January 31, 2019.
As for November 26, 2018, creators are no longer able to create or edit video credits, however, credits in existing videos will remain available until discontinued on January 2019. Credits were available for channels with at least 5,000 subscribers.
In the past, credits provided a way for creators to tag collaborators on projects and link to each individual channel, in different areas, such as video stars, writers, musicians, and others.
That being said, YouTube does note that you can still get credit for subtitle & CC contributions.
For more information about the video credits feature, visit YouTube's official article.