Development recommendation Fiori in S/4HANA?

Hello All,

Quick question regarding UI for underlying ABAP developments in S/4HANA.

Is Fiori the only development technology for custom ABAP applications in S/4HANA on-premise? Is Fiori based UI developments a must?

Even if it is not recommended, is it still possible to work with SAP GUI's and Classic dynpros?

Thanks and Regards