I have no idea why they disabled byproducts explicitly for subcontracting.

But you can disable this constraint in mrp_subcontracting

​ ​ ​ class MrpBom(models.Model):   
_inherit = 'mrp.bom'       

​@api.constrains('operation_ids', 'byproduct_ids', 'type')
​def _check_subcontracting_no_operation(self):    ​
​ ​return True ​

You would afterwards need some push rule for the byproducts