wp-includes/style-engine/class-wp-style-engine-css-rule.php:135Gets the rules group.
string public function get_rules_group() { return $this->rules_group; }Introduced in 6.6.0. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/style-engine/class-wp-style-engine-css-rule.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.