Add deprecation disclaimer to dataset card

README.md

@@ -139,6 +139,10 @@ task_ids:
 paperswithcode_id: mc4
 ---
 
+<div class="course-tip course-tip-orange bg-gradient-to-br dark:bg-gradient-to-r before:border-orange-500 dark:before:border-orange-800 from-orange-50 dark:from-gray-900 to-white dark:to-gray-950 border border-orange-50 text-orange-700 dark:text-gray-400">
+  <p><b>Deprecated:</b> Dataset "mc4" is deprecated and will be deleted. Use "<a href="https://huggingface.co/datasets/allenai/c4">allenai/c4</a>" instead.</p>
+</div>
+
 # Dataset Card for mC4
 
 ## Table of Contents