[RN][iOS] Remove the legacy arch by default

packages/react-native/Package.swift

@@ -923,6 +923,7 @@ extension Target {
         .define("DEBUG", .when(configuration: .debug)),
         .define("NDEBUG", .when(configuration: .release)),
         .define("USE_HERMES", to: "1"),
+        .define("RCT_REMOVE_LEGACY_ARCH", to: "1"),
       ] + defines + cxxCommonHeaderPaths
 
     return .target(

packages/react-native/scripts/react_native_pods.rb

@@ -88,6 +88,9 @@ def use_react_native! (
   # Users can still turn them off and build from source by setting the environment variable to 0.
   ENV['RCT_USE_RN_DEP'] = ENV['RCT_USE_RN_DEP'] == '0' ? '0' : '1'
   ENV['RCT_USE_PREBUILT_RNCORE'] = ENV['RCT_USE_PREBUILT_RNCORE'] == '0' ? '0' : '1'
+  # Make `REMOVE_LEGACY_ARCH` enabled by default. This will build React Native
+  # excluding the legacy arch unless the user turns this flag off explicitly.
+  ENV['RCT_REMOVE_LEGACY_ARCH'] = ENV['RCT_REMOVE_LEGACY_ARCH'] == '0' ? '0' : '1'
 
   ReactNativePodsUtils.check_minimum_required_xcode()