/**
 * Add custom 'visibleOnMobile' attribute to the core/group block.
 *
 * @param array $metadata Metadata for registering a block type.
 * @return array
 */
function my_filter_block_type_metadata( $metadata ) {
	if ( 'core/group' === $metadata['name'] ) {
		$metadata['attributes']['visibleOnMobile'] = array(
			'type'    => 'boolean',
			'default' => false,
		);
	}
	return $metadata;
}
add_filter( 'block_type_metadata', 'my_filter_block_type_metadata' );