RPC/encoded is a vestige from before SOAP objects were defined with XML Schema. It’s not widely supported anymore. You will need to generate the stubs using Apache Axis 1.0, which is from the same era.
java org.apache.axis.wsdl.WSDL2Java http://someurl?WSDL
You will need the following jars or equivalents in the -cp classpath param:
This will generate similar stubs to wsimport.
Alternatively, if you are not using the parts of the schema that require rpc/encoded, you can download a copy of the WSDL and comment out those bits. Then run wsimport against the local file.
If you look at the WSDL, the following bits are using rpc/encoded: