The @api JSDoc tag has been used in the library code since the initial commit. However, I have not been able to find any documentation on it. The fact its values are properly highlighted in VS Code is due to microsoft/vscode@9ad4bcd, which includes no explanation on why the highlighting was added. My best guess is that it was added just to cover de facto usage like that in Commander (see microsoft/vscode#189812).

The well-documented alternatives to @api private (which is currently used throughout the library) are @private and @access private. Should we use one of them instead?