GREAT NEWS… I can confirm it’s back up and running and working on my site once more. I emailed Shopify and got the below reply. I actually didn’t need to do anything that they suggested, as it was already working. So relieved.
Shopify Suport Reply…
Thank you so much for reaching out about this! We were experiencing an incident where this feature was removed as an option. However, our developers have been hard at work and a fix has been implemented. you should see this feature back up and functioning as expected if not already then very shortly here. You may need to clear your browsers cache and cookies in order to see the update. If you are finding that you are not seeing the update shortly, please try a cache and cookie clear. I have included a link below to show you how 
Clear your cache and cookies on your browser. After this, completely reboot the browser.