diff --git a/CMakeLists.txt b/CMakeLists.txt
index 9fe1135ff0..3acdc39334 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -196,12 +196,6 @@ list(APPEND _Foundation_common_build_flags
     "-Wno-switch"
     "-fblocks")
 
-if(NOT CMAKE_SYSTEM_NAME STREQUAL "WASI")
-    list(APPEND _Foundation_common_build_flags
-        "-DDEPLOYMENT_ENABLE_LIBDISPATCH"
-        "-DSWIFT_CORELIBS_FOUNDATION_HAS_THREADS")
-endif()
-
 if(NOT "${CMAKE_C_SIMULATE_ID}" STREQUAL "MSVC")
     list(APPEND _Foundation_common_build_flags
         "-fconstant-cfstrings"
@@ -245,10 +239,6 @@ if(CMAKE_SYSTEM_NAME STREQUAL "WASI")
         list(APPEND _Foundation_swift_build_flags "SHELL:-Xcc -D${def}")
         list(APPEND _Foundation_common_build_flags "-D${def}")
     endforeach()
-else()
-    # Assume we have threads on other platforms
-    list(APPEND _Foundation_swift_build_flags
-        "-DSWIFT_CORELIBS_FOUNDATION_HAS_THREADS")
 endif()
 
 if(CMAKE_SYSTEM_NAME STREQUAL "Linux" OR CMAKE_SYSTEM_NAME STREQUAL "Android")
diff --git a/Package.swift b/Package.swift
index c9114d6936..9c9cb72b0a 100644
--- a/Package.swift
+++ b/Package.swift
@@ -3,18 +3,6 @@
 
 import PackageDescription
 
-let platformsWithThreads: [Platform] = [
-    .iOS,
-    .macOS,
-    .tvOS,
-    .watchOS,
-    .macCatalyst,
-    .driverKit,
-    .android,
-    .linux,
-    .windows,
-]
-
 var dispatchIncludeFlags: [CSetting] = []
 if let environmentPath = Context.environment["DISPATCH_INCLUDE_PATH"] {
     dispatchIncludeFlags.append(.unsafeFlags([
@@ -92,10 +80,8 @@ let coreFoundationBuildSettings: [CSetting] = [
     .define("DEBUG", .when(configuration: .debug)),
     .define("CF_BUILDING_CF"),
     .define("CF_WINDOWS_EXECUTABLE_INITIALIZER", .when(platforms: [.windows])), // Ensure __CFInitialize is run even when statically linked into an executable
-    .define("DEPLOYMENT_ENABLE_LIBDISPATCH", .when(platforms: platformsWithThreads)),
     .define("DEPLOYMENT_RUNTIME_SWIFT"),
     .define("HAVE_STRUCT_TIMESPEC"),
-    .define("SWIFT_CORELIBS_FOUNDATION_HAS_THREADS", .when(platforms: platformsWithThreads)),
     .define("_GNU_SOURCE", .when(platforms: [.linux, .android])),
     .define("_WASI_EMULATED_SIGNAL", .when(platforms: [.wasi])),
     .unsafeFlags([
@@ -124,9 +110,7 @@ let interfaceBuildSettings: [CSetting] = [
     .headerSearchPath("../CoreFoundation/internalInclude"),
     .define("DEBUG", .when(configuration: .debug)),
     .define("CF_BUILDING_CF"),
-    .define("DEPLOYMENT_ENABLE_LIBDISPATCH"),
     .define("HAVE_STRUCT_TIMESPEC"),
-    .define("SWIFT_CORELIBS_FOUNDATION_HAS_THREADS", .when(platforms: platformsWithThreads)),
     .define("_GNU_SOURCE", .when(platforms: [.linux, .android])),
     .define("_WASI_EMULATED_SIGNAL", .when(platforms: [.wasi])),
     .unsafeFlags([
@@ -149,7 +133,6 @@ let interfaceBuildSettings: [CSetting] = [
 
 let swiftBuildSettings: [SwiftSetting] = [
     .define("DEPLOYMENT_RUNTIME_SWIFT"),
-    .define("SWIFT_CORELIBS_FOUNDATION_HAS_THREADS"),
     .swiftLanguageMode(.v6),
     .unsafeFlags([
         "-Xfrontend",