diff --git a/libcxx/docs/ReleaseNotes.rst b/libcxx/docs/ReleaseNotes.rst
index 604e2369c2022..c01332696218c 100644
--- a/libcxx/docs/ReleaseNotes.rst
+++ b/libcxx/docs/ReleaseNotes.rst
@@ -96,5 +96,3 @@ ABI Affecting Changes
 
 Build System Changes
 --------------------
-- Support for ``libcxx``, ``libcxxabi`` and ``libunwind`` in ``LLVM_ENABLE_PROJECTS`` has officially
-  been removed. Instead, please build according to :ref:`these instructions <build instructions>`.
diff --git a/libcxx/utils/ci/buildkite-pipeline.yml b/libcxx/utils/ci/buildkite-pipeline.yml
index 778daea1c4ada..cec9d21612197 100644
--- a/libcxx/utils/ci/buildkite-pipeline.yml
+++ b/libcxx/utils/ci/buildkite-pipeline.yml
@@ -383,6 +383,23 @@ steps:
             limit: 2
       timeout_in_minutes: 120
 
+    - label: "Legacy LLVM_ENABLE_PROJECTS build"
+      command: "libcxx/utils/ci/run-buildbot legacy-project-build"
+      artifact_paths:
+        - "**/test-results.xml"
+        - "**/*.abilist"
+      env:
+          CC: "clang-${LLVM_HEAD_VERSION}"
+          CXX: "clang++-${LLVM_HEAD_VERSION}"
+      agents:
+        queue: "libcxx-builders"
+        os: "linux"
+      retry:
+        automatic:
+          - exit_status: -1  # Agent was lost
+            limit: 2
+      timeout_in_minutes: 120
+
   # Tests with various build configurations.
   - label: "Static libraries"
     command: "libcxx/utils/ci/run-buildbot generic-static"
diff --git a/libcxx/utils/ci/run-buildbot b/libcxx/utils/ci/run-buildbot
index ec5c02b406bda..91f203a6d4e01 100755
--- a/libcxx/utils/ci/run-buildbot
+++ b/libcxx/utils/ci/run-buildbot
@@ -502,6 +502,21 @@ legacy-test-config)
                    -DLIBUNWIND_TEST_CONFIG="${MONOREPO_ROOT}/libunwind/test/lit.site.cfg.in"
     check-runtimes
 ;;
+legacy-project-build)
+    clean
+
+    echo "--- Generating CMake"
+    ${CMAKE} \
+          -S "${MONOREPO_ROOT}/llvm" \
+          -B "${BUILD_DIR}" \
+          -DLLVM_ENABLE_PROJECTS="libcxx;libunwind;libcxxabi" \
+          -GNinja -DCMAKE_MAKE_PROGRAM="${NINJA}" \
+          -DCMAKE_BUILD_TYPE=RelWithDebInfo \
+          -DCMAKE_INSTALL_PREFIX="${INSTALL_DIR}" \
+          -DLLVM_LIT_ARGS="-sv --show-unsupported --xunit-xml-output test-results.xml --timeout=1500" \
+          -DLIBCXX_CXX_ABI=libcxxabi
+    check-runtimes
+;;
 aarch64)
     clean
     generate-cmake -C "${MONOREPO_ROOT}/libcxx/cmake/caches/AArch64.cmake"
diff --git a/llvm/CMakeLists.txt b/llvm/CMakeLists.txt
index c60e34efa9b34..571f82d54b36d 100644
--- a/llvm/CMakeLists.txt
+++ b/llvm/CMakeLists.txt
@@ -109,7 +109,7 @@ endif()
 # LLVM_EXTERNAL_${project}_SOURCE_DIR using LLVM_ALL_PROJECTS
 # This allows an easy way of setting up a build directory for llvm and another
 # one for llvm+clang+... using the same sources.
-set(LLVM_ALL_PROJECTS "bolt;clang;clang-tools-extra;compiler-rt;cross-project-tests;libc;libclc;lld;lldb;mlir;openmp;polly;pstl")
+set(LLVM_ALL_PROJECTS "bolt;clang;clang-tools-extra;compiler-rt;cross-project-tests;libc;libclc;libcxx;libcxxabi;libunwind;lld;lldb;mlir;openmp;polly;pstl")
 # The flang project is not yet part of "all" projects (see C++ requirements)
 set(LLVM_EXTRA_PROJECTS "flang")
 # List of all known projects in the mono repo
@@ -122,7 +122,13 @@ if( LLVM_ENABLE_PROJECTS STREQUAL "all" )
 endif()
 foreach(proj ${LLVM_ENABLE_PROJECTS})
   if (NOT proj STREQUAL "llvm" AND NOT "${proj}" IN_LIST LLVM_KNOWN_PROJECTS)
-     MESSAGE(FATAL_ERROR "${proj} isn't a known project: ${LLVM_KNOWN_PROJECTS}. Did you mean to enable it as a runtime in LLVM_ENABLE_RUNTIMES?")
+     MESSAGE(FATAL_ERROR "${proj} isn't a known project: ${LLVM_KNOWN_PROJECTS}")
+  endif()
+endforeach()
+foreach(proj "libcxx" "libcxxabi" "libunwind")
+  if (${proj} IN_LIST LLVM_ENABLE_PROJECTS)
+    message(WARNING "Using LLVM_ENABLE_PROJECTS=${proj} is deprecated now, please use -DLLVM_ENABLE_RUNTIMES=${proj} or "
+                    "see the instructions at https://libcxx.llvm.org/BuildingLibcxx.html for building the runtimes.")
   endif()
 endforeach()