Apple disallowed HTTP by default, you can flip a bit in the config to allowlist some/all endpoints to HTTP. Not clear what the App Store actually does when reviewing this info when you submit.
[1] https://www.nowsecure.com/blog/2017/12/29/enable-ios-app-tra...
[2] https://developer.apple.com/news/?id=12212016b
[3] https://www.klundberg.com/blog/app-transport-security-delay/