Hooking into page specific instance of checkout flow form
Conditionals are still the answer, but turns out there's an easier way that doesn't involve getting the current path:

function MYMODULE_form_commerce_checkout_flow_multistep_default_alter(&amp;$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {
  if ($form['#step_id'] == 'order_information') {
    Do stuff to the order information page form.
  }
  if ($form['#step_id'] == 'review') {
    Do stuff to the review page form.
  }
}