wp-includes/class-wp-roles.php:342Gets the ID of the site for which roles are currently initialized.
int public function get_site_id() { return $this->site_id; }Introduced in 4.9.0. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/class-wp-roles.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.