Source

File: wp-includes/ms-functions.php

function get_subdirectory_reserved_names() {
	$names = array(
		'page',
		'comments',
		'blog',
		'files',
		'feed',
		'wp-admin',
		'wp-content',
		'wp-includes',
		'wp-json',
		'embed',
	);

	/**
	 * Filters reserved site names on a sub-directory Multisite installation.
	 *
	 * @since 3.0.0
	 * @since 4.4.0 'wp-admin', 'wp-content', 'wp-includes', 'wp-json', and 'embed' were added
	 *              to the reserved names list.
	 *
	 * @param string[] $subdirectory_reserved_names Array of reserved names.
	 */
	return apply_filters( 'subdirectory_reserved_names', $names );
}