Added a note for deprecation instead of removing the whole section

Author: MaryNfs

public/content/developers/docs/apis/json-rpc/index.md

@@ -383,6 +383,33 @@ curl -X POST --data '{"jsonrpc":"2.0","method":"eth_syncing","params":[],"id":1}
 }
 ```
 
+### eth_coinbase {#eth_coinbase}
+
+Returns the client coinbase address.
+
+> **Note:** This method has been deprecated as of **v1.14.0** and is no longer supported. Attempting to use this method will result in a "Method not supported" error.
+
+**Parameters**
+
+None
+
+**Returns**
+
+`DATA`, 20 bytes - the current coinbase address.
+
+**Example**
+
+```js
+// Request
+curl -X POST --data '{"jsonrpc":"2.0","method":"eth_coinbase","params":[],"id":64}'
+// Result
+{
+  "id":64,
+  "jsonrpc": "2.0",
+  "result": "0x407d73d8a49eeb85d32cf465507dd71d507100c1"
+}
+```
+
 ### eth_chainId {#eth_chainId}
 
 Returns the chain ID used for signing replay-protected transactions.