The problem that POSIX does not define the behavior of link() regarding symlinks (and that Unix implementations differ indeed), is independent from Python's os.link() defaults.

Since it makes no sense to call link(), when linkat() is available, I propose this change:

--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -3512,15 +3512,11 @@ os_link_impl(PyObject *module, path_t *src, path_t *dst, int src_dir_fd,
 #else
     Py_BEGIN_ALLOW_THREADS
 #ifdef HAVE_LINKAT
-    if ((src_dir_fd != DEFAULT_DIR_FD) ||
-        (dst_dir_fd != DEFAULT_DIR_FD) ||
-        (!follow_symlinks))
-        result = linkat(src_dir_fd, src->narrow,
-            dst_dir_fd, dst->narrow,
-            follow_symlinks ? AT_SYMLINK_FOLLOW : 0);
-    else
+    result = linkat(src_dir_fd, src->narrow, dst_dir_fd, dst->narrow,
+                    follow_symlinks ? AT_SYMLINK_FOLLOW : 0);
+#else
+    result = link(src->narrow, dst->narrow);
 #endif /* HAVE_LINKAT */
-        result = link(src->narrow, dst->narrow);
     Py_END_ALLOW_THREADS

     if (result)


This fix also simplifies the code, and should be safe for back-porting.

Whether Python's defaults should be changed regarding Windows and those (mostly obsolete) Unix platforms that do not provide linkat(), is another question.