Possible to Find Which IdP Within hook_saml_sp_drupal_login_user_attributes_alter?

Well... it's not pretty, but I ended up placing the IdP clas in the session.

I created a class MyModuleSamlSPDrupalLoginController that extended SamlSPDrupalLoginController

Stored $idp in the session:

public function initiate(Idp $idp) {
  $_SESSION['my_module_idp'] = $idp;
  ...

And altered the login route:

protected function alterRoutes(RouteCollection $collection) {
  if($route = $collection->get('saml_sp_drupal_login.login')) {
    $route->setDefault('_controller', '\Drupal\stv_saml\Controller\StvSamlSPDrupalLoginController::initiate');
  }
}