diff --git a/libcxx/docs/ReleaseNotes.rst b/libcxx/docs/ReleaseNotes.rst
--- a/libcxx/docs/ReleaseNotes.rst
+++ b/libcxx/docs/ReleaseNotes.rst
@@ -96,3 +96,5 @@
 
 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
--- a/libcxx/utils/ci/buildkite-pipeline.yml
+++ b/libcxx/utils/ci/buildkite-pipeline.yml
@@ -386,23 +386,6 @@
             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
--- a/libcxx/utils/ci/run-buildbot
+++ b/libcxx/utils/ci/run-buildbot
@@ -502,21 +502,6 @@
                    -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
--- a/llvm/CMakeLists.txt
+++ b/llvm/CMakeLists.txt
@@ -109,7 +109,7 @@
 # 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;libcxx;libcxxabi;libunwind;lld;lldb;mlir;openmp;polly;pstl")
+set(LLVM_ALL_PROJECTS "bolt;clang;clang-tools-extra;compiler-rt;cross-project-tests;libc;libclc;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
@@ -122,13 +122,7 @@
 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: ${LLVM_KNOWN_PROJECTS}")
-  endif()
-endforeach()
-foreach(proj "libcxx" "libcxxabi" "libunwind")
-  if (${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.")
+     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?")
   endif()
 endforeach()