They are most likely using one-click unsubscribe. In early 2024, Google and Yahoo mandated that bulk emails support them, so CiviCRM added them. So there's an "unsubscribe" link at the top of these emails in GMail.

I wrote an extension to remove them if you disable the requirement for an unsubscribe link. I haven't published it, but I could if that's helpful. But for the techies, the code itself is right here:

function nounsubheader_civicrm_alterMailParams(&$params, $context) {
  // Don't add "List-Unsubscribe" header if the mailing doesn't have an unsubscribe token.
  if (Civi::settings()->get('disable_mandatory_tokens_check')) {
    unset($params['List-Unsubscribe']);
  }
}

They are most likely using one-click unsubscribe. In early 2024, Google and Yahoo mandated that bulk emails support them, so CiviCRM added them. So there's an "unsubscribe" link at the top of these emails in GMail.

I wrote an extension to remove them if you disable the requirement for an unsubscribe link. I haven't published it, but I could if that's helpful. But for the techies, the code itself is right here:

function nounsubheader_civicrm_alterMailParams(&$params, $context) {
  // Don't add "List-Unsubscribe" header if the mailing doesn't have an unsubscribe token.
  if (Civi::settings()->get('disable_mandatory_tokens_check')) {
    unset($params['List-Unsubscribe']);
  }
}

UPDATE: It seems there's an easier option now - Administer » CiviMail » Mailer Settings, looks like you can remove HTTP(S) One-Click from the Unsubscribe Methods.