Yes. There are two ways to live stream to an unlisted YouTube event.
Option 1: Stream to an unlisted YouTube event using RTMP
First, create an Unlisted live event in YouTube Studio. Then, copy the Stream URL and Stream Key, and connect it to Riverside using Custom RTMP.
Option 2: Schedule in Riverside and change visibility in YouTube Studio
Follow the steps in Live stream to YouTube to create and connect your live stream in Riverside.
Once the event appears in YouTube Studio, open it and change the visibility to Unlisted.
Note: Riverside only offers Public and Private visibility during setup.