Add comment about retaining feature activation slot logic

rpc/src/rpc.rs

@@ -625,6 +625,10 @@ impl JsonRpcRequestProcessor {
         // Determine if partitioned epoch rewards were enabled for the desired
         // epoch
         let bank = self.get_bank_with_config(context_config)?;
+
+        // DO NOT CLEAN UP with feature_set::enable_partitioned_epoch_reward
+        // This logic needs to be retained indefinitely to support historical
+        // rewards before and after feature activation.
         let partitioned_epoch_reward_enabled_slot = bank
             .feature_set
             .activated_slot(&feature_set::enable_partitioned_epoch_reward::id());