The SIM module provided in Zen Cart payments is the one that instead of processing on your website, the buyer is redirected to an authorize.net page to finalize their payment and then returned to your website.
As of August 15, they are requiring an accurate listing of the Relay Response URL. This is not hard to do and I think should be already set up for your account; however, I do believe it has been working without this as the one account I looked at did not have a relay response url set up.
I rarely see users who have the SIM method set up and recommend not using nowadays because folks are used to paying on a website and not being redirected. The redirect can stop a buyer from finishing his purchase as one of the warnings of site security good practices is to beware of links that redirect the buyer to another page.
But I do know they are sending emails to those who might be impacted by this change. It is entitled “Security Update – Relay Response White Listing”. Please note that if you did receive this email today, Aug 8, it also says the domains from June 2019 will be listed in your account. I did just speak with Authorize.net and I was told that list will not be available until Aug 15. I confirmed this in several ways to be sure as it makes no sense to me to wait.
SIM setup instructions include how to do this: https://www.zen-cart.com/content.php?303-how-to-set-up-the-authorizenet-sim-payment-module so you can check this ahead of the deadline or after you start having issues. Failure to do this correctly will completely stop your cart from taking credit cards.
Not sure which method you are using? You can see they are listed as SIM and AIM. It is only the SIM this applies to and the button at the right must be green for you to be actually using that module.
Need help? Just give me a shout via email if you think you are using SIM and need someone to check this out for you.