Windows Terminal – change default window split mode (alt+shift+d) to down/horizontal

Open settings.json and add to actions:

    "actions": 
    [
        {
            "command": 
            {
                "action": "splitPane",
                "split": "down",
                "splitMode": "duplicate"
            },
            "id": "User.splitPane.10B260D2"
        }
    ],Code language: JavaScript (javascript)

add to keybindings section:

    "keybindings": 
    [
        ...,
        {
            "id": "User.splitPane.10B260D2",
            "keys": "alt+shift+d"
        }
    ],Code language: JavaScript (javascript)

Now, after pressing alt+shift+d: