Starting from YouTrack 2017.2 you can enable experimental features for your InCloud instance or Standalone installation. These features are in early or intermediate stages of development. This strategy lets us get feedback for a feature as we work on it, learning more about what you want to see in the final product.
If you are an administrator, simply enable the features you want to try on the Feature Configuration page. This page shows the features that you can enable system-wide and other features that you can enable for specific groups.
YouTrack 2017.2 introduces the first version of the activity stream. This functionality replaces the tab interface in issue views for comments, history, linked issues, similar issues, time tracking, and VCS changes. These components are merged into a single stream that you can filter to display specific types of activities. In the current version, you can add work item and comments in the issue preview mode on Agile board. You can also apply filters to display either work items or comments separately.