Interfaces and Greenfoot
Yes, Greenfoot certainly has interfaces. I doubt that they would take them out. The people who built greenfoot are likely as appalled as I am about the College Board action on interfaces. But there is no UI element that specifically adds a new interface to a project. 

But they may not appear in the same place that most classes do in the sidebar. Look at the bottom of the list. 

Use the menu option to create a "New Class". Edit the resulting template to change it to a legal interface. Compile and save. This was with v 3.5. 

You can do the same with other "class-like" things such as enumerations. The compiler hiding behind greenfoot is all of Java. 

You can also go to https://greenroom.greenfoot.org and ask such questions there (after joining). 

Also, many of the labs that you find on both the greenfoot and greenroom sites will use interfaces.