wp-includes/SimplePie/src/Restriction.php:112Get the relationship
string|null public function get_relationship() { if ($this->relationship !== null) { return $this->relationship; } return null; }One change between 6.7.7 and 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
string|null to SimplePie\?self::RELATIONSHIP_*.verified against sourcesrc/wp-includes/SimplePie/src/Restriction.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.