diff --git a/clang/lib/Driver/ToolChains/Linux.cpp b/clang/lib/Driver/ToolChains/Linux.cpp
index ceb1a982c3a4..eedbb12d0dd5 100644
--- a/clang/lib/Driver/ToolChains/Linux.cpp
+++ b/clang/lib/Driver/ToolChains/Linux.cpp
@@ -307,13 +307,6 @@ Linux::Linux(const Driver &D, const llvm::Triple &Triple, const ArgList &Args)
 
   Generic_GCC::AddMultiarchPaths(D, SysRoot, OSLibDir, Paths);
 
-  // The deprecated -DLLVM_ENABLE_PROJECTS=libcxx configuration installs
-  // libc++.so in D.Dir+"/../lib/". Detect this path.
-  // TODO Remove once LLVM_ENABLE_PROJECTS=libcxx is unsupported.
-  if (StringRef(D.Dir).startswith(SysRoot) &&
-      D.getVFS().exists(D.Dir + "/../lib/libc++.so"))
-    addPathIfExists(D, D.Dir + "/../lib", Paths);
-
   addPathIfExists(D, concat(SysRoot, "/lib"), Paths);
   addPathIfExists(D, concat(SysRoot, "/usr/lib"), Paths);
 }
diff --git a/compiler-rt/lib/sanitizer_common/symbolizer/scripts/build_symbolizer.sh b/compiler-rt/lib/sanitizer_common/symbolizer/scripts/build_symbolizer.sh
index 0386be5b76e7..3fe8e45bfcbc 100755
--- a/compiler-rt/lib/sanitizer_common/symbolizer/scripts/build_symbolizer.sh
+++ b/compiler-rt/lib/sanitizer_common/symbolizer/scripts/build_symbolizer.sh
@@ -100,12 +100,12 @@ if [[ ! -d ${LIBCXX_BUILD} ]]; then
   mkdir -p ${LIBCXX_BUILD}
   cd ${LIBCXX_BUILD}
   LIBCXX_FLAGS="${FLAGS} -Wno-macro-redefined"
-  PROJECTS=
+  RUNTIMES=
   if [[ ! -d $LLVM_SRC/projects/libcxxabi ]] ; then
-    PROJECTS="-DLLVM_ENABLE_PROJECTS='libcxx;libcxxabi'"
+    RUNTIMES="-DLLVM_ENABLE_RUNTIMES='libcxx;libcxxabi'"
   fi
   cmake -GNinja \
-    ${PROJECTS} \
+    ${RUNTIMES} \
     -DCMAKE_BUILD_TYPE=Release \
     -DCMAKE_C_COMPILER=$CC \
     -DCMAKE_CXX_COMPILER=$CXX \
diff --git a/libcxx/utils/ci/buildkite-pipeline.yml b/libcxx/utils/ci/buildkite-pipeline.yml
index 8e804552f359..bf9db29ce7f5 100644
--- a/libcxx/utils/ci/buildkite-pipeline.yml
+++ b/libcxx/utils/ci/buildkite-pipeline.yml
@@ -321,20 +321,6 @@ 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"
-      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/oss-fuzz.sh b/libcxx/utils/ci/oss-fuzz.sh
index 12149de87ce5..123ac47fc449 100755
--- a/libcxx/utils/ci/oss-fuzz.sh
+++ b/libcxx/utils/ci/oss-fuzz.sh
@@ -13,7 +13,7 @@ INSTALL=cxx_install_dir
 
 mkdir ${BUILD}
 cmake -S ${PWD} -B ${BUILD} \
-      -DLLVM_ENABLE_PROJECTS="libcxx;libcxxabi" \
+      -DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi" \
       -DCMAKE_BUILD_TYPE=RelWithDebInfo \
       -DCMAKE_INSTALL_PREFIX="${INSTALL}"
 cmake --build ${BUILD} --target install-cxx-headers
diff --git a/libcxx/utils/ci/run-buildbot b/libcxx/utils/ci/run-buildbot
index 27431f3c2cac..220082c1209c 100755
--- a/libcxx/utils/ci/run-buildbot
+++ b/libcxx/utils/ci/run-buildbot
@@ -510,21 +510,6 @@ legacy-test-config)
                    -DLIBUNWIND_TEST_CONFIG="${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 7897a919701a..de13ab0df24e 100644
--- a/llvm/CMakeLists.txt
+++ b/llvm/CMakeLists.txt
@@ -123,8 +123,9 @@ foreach(proj ${LLVM_ENABLE_PROJECTS})
 endforeach()
 foreach(proj "libcxx" "libcxxabi" "libunwind")
   if (${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
+      "Using LLVM_ENABLE_PROJECTS=${proj} is incorrect. Please use -DLLVM_ENABLE_RUNTIMES=${proj} or "
+      "see the instructions at https://libcxx.llvm.org/BuildingLibcxx.html for building the runtimes.")
   endif()
 endforeach()