Emit warning for text input. (#11590)

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: trivialfis

src/common/error_msg.cc

@@ -19,16 +19,6 @@ namespace xgboost::error {
   return ss.str();
 }
 
-void WarnDeprecatedGPUHist() {
-  auto msg =
-      "The tree method `gpu_hist` is deprecated since 2.0.0. To use GPU training, set the `device` "
-      R"(parameter to CUDA instead.
-
-    E.g. tree_method = "hist", device = "cuda"
-)";
-  LOG(WARNING) << msg;
-}
-
 void WarnManualUpdater() {
   static std::once_flag flag;
   std::call_once(flag, [] {

src/common/error_msg.h

@@ -84,8 +84,6 @@ inline void WarnOldSerialization() {
   logged = true;
 }
 
-void WarnDeprecatedGPUHist();
-
 void WarnManualUpdater();
 
 void WarnEmptyDataset();

src/data/data.cc

@@ -931,6 +931,11 @@ DMatrix* DMatrix::Load(const std::string& uri, bool silent, DataSplitMode data_s
 
   int partid = 0, npart = 1;
 
+  static std::once_flag warning_flag;
+  std::call_once(warning_flag, []() {
+    LOG(WARNING) << "Text file input has been deprecated since 3.1";
+  });
+
   fname = data::ValidateFileFormat(fname);
   std::unique_ptr<dmlc::Parser<std::uint32_t>> parser(
       dmlc::Parser<std::uint32_t>::Create(fname.c_str(), partid, npart, "auto"));