wp-includes/class-wp-post.php:279Isset-er.
$keystringbool public function __isset( $key ) { if ( 'ancestors' === $key ) { return true; } if ( 'page_template' === $key ) { return true; } if ( 'post_category' === $key ) { return true; } if ( 'tags_input' === $key ) { return true; } return metadata_exists( 'post', $this->ID, $key ); }Introduced in 3.5.0. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/class-wp-post.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.