Load a .mo file for the old textdomain if one exists

textdomain-back-compat.php

/**
 * Load a .mo file for the old textdomain if one exists
 *
 * If you have to change your textdomain to comply with upcoming
 * plugin and theme repository standards, this function will check to
 * see if an old translation file exists and load it if it does, so
 * that people don't lose their translations.
 *
 * h/t: https://github.com/10up/grunt-wp-plugin/issues/21#issuecomment-62003284
 */
add_filter( 'load_textdomain_mofile', 'customslug_load_old_textdomain' ), 10, 2 );
function customslug_load_old_textdomain( $mofile, $textdomain ) {

		if ( $textdomain === 'new-textdomain' && 0 === strpos( $mofile, WP_LANG_DIR . '/plugins/'  ) && !file_exists( $mofile ) ) {
			$mofile = dirname( $mofile ) . DIRECTORY_SEPARATOR . str_replace( $textdomain, 'old-textdomain', basename( $mofile ) );
		}

		return $mofile;
}