The key bind for split a vertical pane seems to be wrong on the panes page

You can either create a new vertical or horizontal pane in Windows Terminal. Splitting vertically will open a new pane to the right of the focused pane and splitting horizontally will open a new pane below the focused pane. To create a new vertical pane of your default profile, you can press the Alt+Shift+= key combination. For a horizontal pane of your default profile, you can use Alt+Shift+-.

The one that worked for me was Alt+Shift+plus as mentioned in the getting started page

You can run multiple shells side-by-side using panes. To open a pane, you can use alt+shift+plus for a vertical pane or alt+shift+minus for a horizontal one. You can also use alt+shift+d to open a duplicate pane of your focused profile. Learn more about panes on the Panes page.

Document Details

⚠ Do not edit this section. It is required for docs.microsoft.com ➟ GitHub issue linking.

• ID: ae7015da-46b8-2579-690c-c83016d8be22
• Version Independent ID: f27e9635-b4a3-3b7f-f7e7-bd0aca099014
• Content: Windows Terminal Panes
• Content Source: TerminalDocs/panes.md
• Product: dev-environment
• Technology: windows-terminal
• GitHub Login: @cinnamon-msft
• Microsoft Alias: cinnamon