PJAMM will ride and document this climb in June, 2018. We have confirmed with several Strava members who have ridden this climb that it can be done on a road bike:
Aryanto Nugroho Yes. You can ride your bike from Banyuwangi to the gate of Ijen Crater, it is well paved. The hardest and technical segment will be the Erek-erek climb (up to 23%).
Aiman Cahyadi Yes this is route to kawah ijen, gradient to the top maximum is 28%
Chris Glasby yes can be ridden on a road bike but you will need at least 34 /30 gearing