Yes, that's the "replacement" cmdlet. And yes, the documentation leaves a lot to be desired, but that's something you have to learn to live with, then it comes to the Graph SDK :(
Here's an example on how to use it:
New-MgDeviceRegisteredOwnerByRef -DeviceId dd16bf84-xxxx-xxxx-xxxx-89d42328529e -OdataId "https://graph.microsoft.com/v1.0/directoryObjects/e0d7442c-xxxx-xxxx-xxxx-ec9887816677"
where you have to specify the device ID and the desired user's ID. The "https://graph.microsoft.com/v1.0/directoryObjects/" prefix is mandatory, as those cmdlets are all autogenerated and any concept of convenience is foreign to them.
An alternative use of the cmdlet is:
New-MgDeviceRegisteredOwnerByRef -DeviceId dd16bf84-xxxx-xxxx-xxxx-89d42328529e -BodyParameter @{"@odata.id" = "https://graph.microsoft.com/v1.0/directoryObjects/fe506ef0-xxxx-xxxx-xxxx-e82f833c3e91"}