There is a new warning for people using S3 without a CDN that I merged yesterday. Looks like I got the error message wrong 
.
Will be fixed by
https://github.com/discourse/discourse/pull/11603
@Richie are you deliberately using S3 without a CDN? I added this warning because we got multiple topics in Meta about site admins paying way too much money for S3 because of the egress traffic costs.