diff --git a/libcxx/CMakeLists.txt b/libcxx/CMakeLists.txt
index 574b262018cd3..95a7d10f055ea 100644
--- a/libcxx/CMakeLists.txt
+++ b/libcxx/CMakeLists.txt
@@ -45,10 +45,6 @@ include(CMakeDependentOption)
 include(HandleCompilerRT)
 
 # Basic options ---------------------------------------------------------------
-option(LIBCXX_ENABLE_ASSERTIONS
-  "Enable assertions inside the compiled library, and at the same time make it the
-   default when compiling user code. Note that assertions can be enabled or disabled
-   by users in their own code regardless of this option." OFF)
 option(LIBCXX_ENABLE_SHARED "Build libc++ as a shared library." ON)
 option(LIBCXX_ENABLE_STATIC "Build libc++ as a static library." ON)
 option(LIBCXX_ENABLE_FILESYSTEM
@@ -759,9 +755,9 @@ config_define_if_not(LIBCXX_ENABLE_WIDE_CHARACTERS _LIBCPP_HAS_NO_WIDE_CHARACTER
 config_define_if_not(LIBCXX_ENABLE_TIME_ZONE_DATABASE _LIBCPP_HAS_NO_TIME_ZONE_DATABASE)
 config_define_if_not(LIBCXX_ENABLE_VENDOR_AVAILABILITY_ANNOTATIONS _LIBCPP_HAS_NO_VENDOR_AVAILABILITY_ANNOTATIONS)
 
+# TODO: Remove in LLVM 21. We're leaving an error to make this fail explicitly.
 if (LIBCXX_ENABLE_ASSERTIONS)
-  message(DEPRECATION "LIBCXX_ENABLE_ASSERTIONS is deprecated and will be removed in LLVM 20. Please use LIBCXX_HARDENING_MODE instead.")
-  set(LIBCXX_HARDENING_MODE "extensive")
+  message(FATAL_ERROR "LIBCXX_ENABLE_ASSERTIONS has been removed. Please use LIBCXX_HARDENING_MODE instead.")
 endif()
 if (LIBCXX_HARDENING_MODE STREQUAL "none")
   config_define(2 _LIBCPP_HARDENING_MODE_DEFAULT)
diff --git a/libcxx/docs/ReleaseNotes/20.rst b/libcxx/docs/ReleaseNotes/20.rst
index 39546493ae8d6..91da1bfe59c78 100644
--- a/libcxx/docs/ReleaseNotes/20.rst
+++ b/libcxx/docs/ReleaseNotes/20.rst
@@ -63,8 +63,9 @@ Improvements and New Features
 Deprecations and Removals
 -------------------------
 
-- TODO: The ``LIBCXX_ENABLE_ASSERTIONS`` CMake variable and the ``_LIBCPP_ENABLE_ASSERTIONS`` macro that were used to
-  enable the safe mode will be removed in LLVM 20.
+- The ``LIBCXX_ENABLE_ASSERTIONS`` CMake variable and the ``_LIBCPP_ENABLE_ASSERTIONS`` macro that were used to
+  enable the safe mode have been removed in LLVM 20. Please use :ref:`support for hardening <using-hardening-modes>`
+  instead.
 
 - Support for the C++20 synchronization library (``<barrier>``, ``<latch>``, ``atomic::wait``, etc.) has been
   removed in language modes prior to C++20. If you are using these features prior to C++20, you will need to
@@ -90,12 +91,6 @@ Deprecations and Removals
 Upcoming Deprecations and Removals
 ----------------------------------
 
-LLVM 20
-~~~~~~~
-
-- TODO
-
-
 LLVM 21
 ~~~~~~~
 
diff --git a/libcxx/include/__config b/libcxx/include/__config
index 4155fd33dabc4..618e60446b6a7 100644
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -39,16 +39,9 @@
 
 // HARDENING {
 
-// This is for backward compatibility -- make enabling `_LIBCPP_ENABLE_ASSERTIONS` (which predates hardening modes)
-// equivalent to setting the extensive mode. This is deprecated and will be removed in LLVM 20.
+// TODO: Remove in LLVM 21. We're making this an error to catch folks who might not have migrated.
 #  ifdef _LIBCPP_ENABLE_ASSERTIONS
-#    warning "_LIBCPP_ENABLE_ASSERTIONS is deprecated, please use _LIBCPP_HARDENING_MODE instead"
-#    if _LIBCPP_ENABLE_ASSERTIONS != 0 && _LIBCPP_ENABLE_ASSERTIONS != 1
-#      error "_LIBCPP_ENABLE_ASSERTIONS must be set to 0 or 1"
-#    endif
-#    if _LIBCPP_ENABLE_ASSERTIONS
-#      define _LIBCPP_HARDENING_MODE _LIBCPP_HARDENING_MODE_EXTENSIVE
-#    endif
+#    error "_LIBCPP_ENABLE_ASSERTIONS has been removed, please use _LIBCPP_HARDENING_MODE instead"
 #  endif
 
 // The library provides the macro `_LIBCPP_HARDENING_MODE` which can be set to one of the following values:
diff --git a/libcxx/test/libcxx/assertions/modes/enabling_assertions_enables_extensive_mode.pass.cpp b/libcxx/test/libcxx/assertions/modes/enabling_assertions_enables_extensive_mode.pass.cpp
deleted file mode 100644
index c496fc32dc939..0000000000000
--- a/libcxx/test/libcxx/assertions/modes/enabling_assertions_enables_extensive_mode.pass.cpp
+++ /dev/null
@@ -1,35 +0,0 @@
-//===----------------------------------------------------------------------===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-
-// TODO(hardening): remove in LLVM 20.
-// This test ensures that enabling assertions with the legacy `_LIBCPP_ENABLE_ASSERTIONS` now enables the extensive
-// hardening mode.
-
-// `check_assertion.h` is only available starting from C++11 and requires Unix headers and regex support.
-// REQUIRES: has-unix-headers
-// UNSUPPORTED: c++03, no-localization
-// The ability to set a custom abort message is required to compare the assertion message (which only happens in the
-// debug mode).
-// XFAIL: libcpp-hardening-mode=debug && availability-verbose_abort-missing
-// HWASAN replaces TRAP with abort or error exit code.
-// XFAIL: hwasan
-// Note that GCC doesn't support `-Wno-macro-redefined`.
-// ADDITIONAL_COMPILE_FLAGS: -U_LIBCPP_HARDENING_MODE -D_LIBCPP_ENABLE_ASSERTIONS=1 -Wno-#warnings -Wno-cpp
-
-#include <cassert>
-#include "check_assertion.h"
-
-int main(int, char**) {
-  static_assert(_LIBCPP_HARDENING_MODE == _LIBCPP_HARDENING_MODE_EXTENSIVE,
-                "The extensive hardening mode should be implicitly enabled");
-
-  _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(true, "Should not fire");
-  TEST_LIBCPP_ASSERT_FAILURE([] { _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(false, "Should fire"); }(), "Should fire");
-
-  return 0;
-}