Can I disable custom keyboards (iOS8) for my app?

I just want to add this for those developers who want to implement this method in Xamarin iOS. The idea is to override theShouldAllowExtensionPointIdentifier method in your AppDelegate:

public override bool ShouldAllowExtensionPointIdentifier(UIApplication application, NSString extensionPointIdentifier)
{
    if (extensionPointIdentifier == UIExtensionPointIdentifier.Keyboard) 
    {           
        return false;
    }
    return true;
}

Swift 3 :

func application(_ application: UIApplication, shouldAllowExtensionPointIdentifier extensionPointIdentifier: UIApplicationExtensionPointIdentifier) -> Bool {
    if extensionPointIdentifier == UIApplicationExtensionPointIdentifier.keyboard {
        return false
    }
    return true
}

Looks like you got what you wanted in beta seed 3. Line 440 of UIApplication.h:

// Applications may reject specific types of extensions based on the extension point identifier.
// Constants representing common extension point identifiers are provided further down.
// If unimplemented, the default behavior is to allow the extension point identifier.
- (BOOL)application:(UIApplication *)application shouldAllowExtensionPointIdentifier:(NSString *)extensionPointIdentifier NS_AVAILABLE_IOS(8_0);

It's not currently included in the docs, but sound like it will do exactly what you asked here.

I'm guessing these "extension point identifiers" are not unique identifiers of extensions, but of their types, as there is also this on line 545:

// Extension point identifier constants
UIKIT_EXTERN NSString *const UIApplicationKeyboardExtensionPointIdentifier NS_AVAILABLE_IOS(8_0);

TLDR: to disable custom keyboards you would include something like this in your app delegate:

- (BOOL)application:(UIApplication *)application shouldAllowExtensionPointIdentifier:(NSString *)extensionPointIdentifier {
    if ([extensionPointIdentifier isEqualToString: UIApplicationKeyboardExtensionPointIdentifier]) {
        return NO;
    }
    return YES;
}

In Swift 5, UIApplicationExtensionPointIdentifier was changed to UIApplication.ExtensionPointIdentifier.

func application(_ application: UIApplication, shouldAllowExtensionPointIdentifier extensionPointIdentifier: UIApplication.ExtensionPointIdentifier) -> Bool {
    if extensionPointIdentifier == UIApplication.ExtensionPointIdentifier.keyboard {
        return false
    }
    return true
}