According to a tweet by an app developer, new apps created with Xamarin (amongst other non-native frameworks) will be rejected beginning early 2019. This was supposedly "announced" through an email about an older app being rejected to the developer.
I have my doubts that this is legitimate - the email's language is not proofread well, and doesn't seem like it would have come from Apple. I also do not see why Xamarin would be banned, considering it is just a binding, or why similar frameworks like React Native would not be banned.
As someone looking into learning Xamarin, this worries me a bit. Some clarification from Apple on whether or not this statement is true would be appreciated.