it is still possible to host your sitemap on external S3 buckets. You must set up a subdomain of your domain pointed as an Alternative Domain Name for your CloudFront distribution, and in order to do this you must go through domain verification with AWS.
Tested codebases are the most signficiant development from Rails and Agile over the last 15 years. End-to-end testing, in particular, is the defining reason why Agile and Rails have been so successful. Not writing tests should be seen as akin to malpractice in software development. Hiring someone to write untested code should be outlawed.