is
Indicates if an interface was compiled with -jvm-default=enable.
In compatibility mode Kotlin/JVM compiler generates method bodies directly in the interface, and also generates bridges in a nested DefaultImpls class. Bridges are intended for use by already existing clients, such as compiled Java code or Kotlin code compiled in the -jvm-default=disable mode.
Also, can be a result of compiling interface with @JvmDefaultWithCompatibility annotation. Check documentation for more details.