Requirements are described at https://developer.apple.com/library/archive/releasenotes/NetworkingInternetWeb/Time_Machine_SMB_Spec/index.html:
SMB protocol version 3.x, including SMB 3.x signing
Handling of SMB2_CREATE_DURABLE_HANDLE_REQUEST_V2 requests, including the Timeout field. For more information, see the Supporting the Timeout Field in SMB2_CREATE_DURABLE_HANDLE_REQUEST_V2 Requests section below.
Handling of SMB2_CREATE_REQUEST_LEASE_V2 requests
Handling of SMB2_CREATE_DURABLE_HANDLE_RECONNECT_V2 requests
Support for Bonjour discovery. For more information, see the Advertising Time Machine Availabilty Through Bonjour section below.
Support for Apple’s F_FULLFSYNC extension to the SMB2 FLUSH command. For more information, see the Supporting the F_FULLFSYNC Extension section below.