Raise deprecation warning

mc4.py

@@ -3,6 +3,7 @@
 
 import gzip
 import json
+import warnings
 
 import datasets
 
@@ -280,6 +281,10 @@ class Mc4(datasets.GeneratorBasedBuilder):
     BUILDER_CONFIG_CLASS = Mc4Config
 
     def _info(self):
+        warnings.warn(
+            "Dataset 'mc4' is deprecated and will be deleted. Use 'allenai/c4' instead.",
+            FutureWarning,
+        )
         return datasets.DatasetInfo(
             description=_DESCRIPTION,
             features=datasets.Features(