The notion of flattenable field has evolved over time with the different models the Valhalla project has implemented. From the access flag JVM_ACC_FLATTENABLE generated by javac, then the JVM_ACC_FIELD_FLATTENABLE VM internal flag computed from the L/Q model, and now in LW3, the notion of flattenable field is quite obsolete (a field with an inline type is always flattenable, no side bit/information, the type of the field says everything).
The current code in the JVM still has remains of the different models and their different implementations, leading to duplications of code or bits to encode the flattenable nature of a field.
It's time to cleanup the code and remove the traces of the old models.