My e-commerce site has gone untouched for a very long time, however sometime in the past few weeks, the iframe suddenly stopped being “allowed”? And obviously CC payments aren’t working. I don’t know exactly what that means. What could have changed?
here is the full url the iframe is trying to load
That’s not a valid link. That link is v0. The supported library is web.squarecdn.com/v1/square.js. 
I literally got that URL from the video provided in the Webpayments SDK How-to section. Mainly because I couldn’t find the URL anywhere else.
I could not find anywhere on the Web Payments SDK Setup Guide where it actually gives the proper URL. That seems like a very large oversight for anyone trying to get started. Am I just blind?
What guide were you looking at? Case it’s in the current guide in step 4 and in the deployment tasks.
- Go to homepage: Developer APIs: Payments, Orders, Customers - Square
- Click SDKs and APIs → Web Payments SDK → Setup Guide
Right, that leads you to the Overview of the Web Payments SDK. The nested specific guides that have the code you add to your application is where the link is documented.