To reproduce:
1. Open users attached project "UnityTlsTest.zip"
2. Open "SampleScene.unity" scene
3. Build for iOS
4. Deploy the Xcode project to an iOS device
5. Observe the Xcode console

Expected result: no errors in the console
Actual result: getting "NotSupportedException: Could not find TLS Provider: `Mono.AppleTls.AppleTlsProvider'" in the Xcode console

Reproduced on:
2018.3.0a1 2018.3.10f1

Not reproduced on:
2018.2.21f1 2019.1.0b8 2019.2.0a9

Devices tested on:
iPhone X iOS 12.1

Notes:
From 2018.3.0a1 to 2018.3.0b5 getting this error in the console and unable to build the project:
"Assets/Scripts/HttpClientRequest.cs(12,27): error CS0246: The type or namespace name 'HttpClient' could not be found (are you missing a using directive or an assembly reference?)"
Changing API Compatibility Level from .NET 4.x to .NET Standard 2.0 fixes the error. But the issue described is still reproducing on an iOS device.
Didn't reproduce on Android, MacOS standalone.