wp-includes/class-wp-duotone.php:651Get the URL for a duotone filter.
$filter_idstringstring private static function get_filter_url( $filter_id ) { return "url(#$filter_id)"; }Introduced in 6.3.0. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/class-wp-duotone.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.