The Split View is no longer a strategy and is on by default if you have the Agenda and/or Day Note strategies enabled.
Click the "Right panel" button on the right of your day view (or press V on your keyboard) to open the right panel.
This will open your agenda or day note depending on which strategies you have enabled.
At the top right of the right panel you will find buttons to edit strategy settings, choose which right panel is open (if you have both the Agenda and Day Note strategies enabled), and close the right panel.
Clicking the list icon lets you choose whether the agenda or day note is open. We will add more right panels here in the future.
Resizing the right panel
If you move your mouse inside the right panel, you will see a small "grabber" along the left edge that you can click and drag to resize the right panel.