JPA Structure was a part of the "JPA Buddy" plugin. When the plugin became a JetBrains pruduct, we merged or eliminated features that duplicated existing IDEA's functionality. Thereby, the "JPA Structure" toolwindow was merged with the "Persistence" one. Now you can find most of the old features there.

Link to doc: https://www.jetbrains.com/help/idea/jpa-buddy-entity-designer.html#jpa-structure