is
Indicates if an interface was compiled with -Xjvm-default=all-compatibility.
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 without all/all-compatibility setting.
Also, can be a result of compiling interface with @JvmDefaultWithCompatibility annotation. Check documentation for more details.