Symlink a plugin in local development, works but got Debug error message – Windows 10

Not sure about the root cause, but you should avoid using relative paths to files. Because they are relative you never sure where exactly a file is being looked after. It might even become a security risk as PHP might look for the file in the default directories.

The proper way is to use the plugin_dir_path, something like

require_once plugin_dir_path(__FILE__).'github-updater.php';