How do you unsplit terminals in VSCode?

Another alternative is to type exit on the terminal you want to close. To switch focus among splitted terminals, you can use alt + left / right arrow, if you prefer not reaching for the mouse.

On Mac, I just need to press Ctrl + D to kill the current terminal.

click on the sub-terminal (added terminal when you split main terminal) and then click on the trash icon. it will only kill that sub-terminal and leave the main terminal.

More ways to unsplit a split terminal WITHOUT terminating either terminal (Version: 1.64.0):

1. Unsplit a split terminal by triggering the Terminal: Unsplit Terminal command from the Command Palette.
   ref: VS Code documentation: Integrated Terminal > Managing Terminals > Grouping

2. From the Terminal Tabs UI on the right side of the Terminal View, right-click the terminal you wish to unsplit, and from the dropdown menu select Unsplit Terminal.