TLDR; They got merged with the "Recommended Tags" section to avoid confusion and provide more powerful results.
One of our earliest Tag related features was "Suggested Tags". A button you could click and view related tags. The most important aspect of this feature is that it showed which keywords people were searching for on YouTube to find your videos (so that you could add them as tags).
Later, we built an inline, more advanced "Recommended Tags" feature. You don't need to click a button to view them - they are automatically displayed below the Tag buttons and include additional useful data such as the Keyword Explorer scores.
Eventually this led to confusion having two features that were so similar. We decided to merge them in order to create a single feature that has the best of both worlds.
You can now find the most important feature from Suggested Tags (how many views were driven to your video through Search), directly in the Related Videos section.