In the old issue view you could enter the ID of a task, in the description or in comments, for example, "please look at WEB-1234" and Jira would automatically create a link to that task and include a nice icon and the status of the referenced task. This doesn't work in the new issue view and I can't find any documentation about it.
The closest I've found is the function to insert a link, but that does not give you the issue status which is nice to have.
