diff --git a/libc/config/config.json b/libc/config/config.json
index 1b0546980e6ba..cfbe9a43948ea 100644
--- a/libc/config/config.json
+++ b/libc/config/config.json
@@ -2,7 +2,7 @@
   "errno": {
     "LIBC_CONF_ERRNO_MODE": {
       "value": "LIBC_ERRNO_MODE_DEFAULT",
-      "doc": "The implementation used for errno, acceptable values are LIBC_ERRNO_MODE_DEFAULT, LIBC_ERRNO_MODE_UNDEFINED, LIBC_ERRNO_MODE_THREAD_LOCAL, LIBC_ERRNO_MODE_SHARED, LIBC_ERRNO_MODE_EXTERNAL, LIBC_ERRNO_MODE_SYSTEM, and LIBC_ERRNO_MODE_SYSTEM_INLINE."
+      "doc": "The implementation used for errno, acceptable values are LIBC_ERRNO_MODE_DEFAULT, LIBC_ERRNO_MODE_UNDEFINED, LIBC_ERRNO_MODE_THREAD_LOCAL, LIBC_ERRNO_MODE_SHARED, LIBC_ERRNO_MODE_EXTERNAL, and LIBC_ERRNO_MODE_SYSTEM_INLINE."
     }
   },
   "threads": {
diff --git a/libc/docs/configure.rst b/libc/docs/configure.rst
index 95c51b8517e64..e23fc824ce7c8 100644
--- a/libc/docs/configure.rst
+++ b/libc/docs/configure.rst
@@ -29,7 +29,7 @@ to learn about the defaults for your platform and target.
     - ``LIBC_CONF_ENABLE_STRONG_STACK_PROTECTOR``: Enable -fstack-protector-strong to defend against stack smashing attack.
     - ``LIBC_CONF_KEEP_FRAME_POINTER``: Keep frame pointer in functions for better debugging experience.
 * **"errno" options**
-    - ``LIBC_CONF_ERRNO_MODE``: The implementation used for errno, acceptable values are LIBC_ERRNO_MODE_DEFAULT, LIBC_ERRNO_MODE_UNDEFINED, LIBC_ERRNO_MODE_THREAD_LOCAL, LIBC_ERRNO_MODE_SHARED, LIBC_ERRNO_MODE_EXTERNAL, LIBC_ERRNO_MODE_SYSTEM, and LIBC_ERRNO_MODE_SYSTEM_INLINE.
+    - ``LIBC_CONF_ERRNO_MODE``: The implementation used for errno, acceptable values are LIBC_ERRNO_MODE_DEFAULT, LIBC_ERRNO_MODE_UNDEFINED, LIBC_ERRNO_MODE_THREAD_LOCAL, LIBC_ERRNO_MODE_SHARED, LIBC_ERRNO_MODE_EXTERNAL, and LIBC_ERRNO_MODE_SYSTEM_INLINE.
 * **"general" options**
     - ``LIBC_ADD_NULL_CHECKS``: Add nullptr checks in the library's implementations to some functions for which passing nullptr is undefined behavior.
 * **"math" options**
diff --git a/libc/src/__support/libc_errno.h b/libc/src/__support/libc_errno.h
index ab5f6a9c4b9d9..3720cdebd5d2a 100644
--- a/libc/src/__support/libc_errno.h
+++ b/libc/src/__support/libc_errno.h
@@ -37,18 +37,11 @@
 // libc doesn't maintain any internal state, instead the embedder must define
 // `int *__llvm_libc_errno(void);` C function.
 #define LIBC_ERRNO_MODE_EXTERNAL 4
-// libc uses system `<errno.h>` `errno` macro directly in the overlay mode; in
-// fullbuild mode, effectively the same as `LIBC_ERRNO_MODE_EXTERNAL`.
-// In this mode, the public C++ symbol `LIBC_NAMESPACE::libc_errno ` is still
-// exported and get redirected to the system `errno` inside its implementation.
-
-// TODO: Investigate deprecating LIBC_ERRNO_MODE_SYSTEM in favor of
-//       LIBC_ERRNO_MODE_SYSTEM_INLINE.
-//       https://github.com/llvm/llvm-project/issues/143454
-#define LIBC_ERRNO_MODE_SYSTEM 5
+// DEPRECATED: #define LIBC_ERRNO_MODE_SYSTEM 5
 // In this mode, the libc_errno is simply a macro resolved to `errno` from the
 // system header <errno.h>.  There is no need to link against the
-// `libc.src.errno.errno` object.
+// `libc.src.errno.errno` object, and public C++ symbol
+// `LIBC_NAMESPACE::libc_errno` doesn't exist.
 #define LIBC_ERRNO_MODE_SYSTEM_INLINE 6
 
 #if !defined(LIBC_ERRNO_MODE) || LIBC_ERRNO_MODE == LIBC_ERRNO_MODE_DEFAULT
@@ -56,7 +49,7 @@
 #if defined(LIBC_FULL_BUILD) || !defined(LIBC_COPT_PUBLIC_PACKAGING)
 #define LIBC_ERRNO_MODE LIBC_ERRNO_MODE_THREAD_LOCAL
 #else
-#define LIBC_ERRNO_MODE LIBC_ERRNO_MODE_SYSTEM
+#define LIBC_ERRNO_MODE LIBC_ERRNO_MODE_SYSTEM_INLINE
 #endif
 #endif // LIBC_ERRNO_MODE
 
@@ -65,7 +58,6 @@
     LIBC_ERRNO_MODE != LIBC_ERRNO_MODE_THREAD_LOCAL &&                         \
     LIBC_ERRNO_MODE != LIBC_ERRNO_MODE_SHARED &&                               \
     LIBC_ERRNO_MODE != LIBC_ERRNO_MODE_EXTERNAL &&                             \
-    LIBC_ERRNO_MODE != LIBC_ERRNO_MODE_SYSTEM &&                               \
     LIBC_ERRNO_MODE != LIBC_ERRNO_MODE_SYSTEM_INLINE
 #error LIBC_ERRNO_MODE must be one of the following values: \
 LIBC_ERRNO_MODE_DEFAULT, \
@@ -73,7 +65,6 @@ LIBC_ERRNO_MODE_UNDEFINED, \
 LIBC_ERRNO_MODE_THREAD_LOCAL, \
 LIBC_ERRNO_MODE_SHARED, \
 LIBC_ERRNO_MODE_EXTERNAL, \
-LIBC_ERRNO_MODE_SYSTEM, \
 LIBC_ERRNO_MODE_SYSTEM_INLINE.
 #endif
 
diff --git a/libc/src/errno/libc_errno.cpp b/libc/src/errno/libc_errno.cpp
index 8ff1eec1b1035..e8960fcd89500 100644
--- a/libc/src/errno/libc_errno.cpp
+++ b/libc/src/errno/libc_errno.cpp
@@ -46,11 +46,6 @@ Errno::operator int() { return shared_errno; }
 void Errno::operator=(int a) { *__llvm_libc_errno() = a; }
 Errno::operator int() { return *__llvm_libc_errno(); }
 
-#elif LIBC_ERRNO_MODE == LIBC_ERRNO_MODE_SYSTEM
-
-void Errno::operator=(int a) { errno = a; }
-Errno::operator int() { return errno; }
-
 #endif
 
 // Define the global `libc_errno` instance.