Mentioned the deprecation of deep parameters in UPGRADE files

UPGRADE-2.8.md

@@ -612,3 +612,21 @@ Yaml
    ```yml
    class: "Foo\\Var"
    ```
+
+HttpFoundation
+--------------
+
+ * Deprecated finding deep items in `ParameterBag::get()`. This may affect you
+   when getting parameters from the `Request` class:
+
+   Before:
+
+   ```php
+   $request->query->get('foo[bar]', null, true);
+   ```
+
+   After:
+
+   ```php
+   $request->query->get('foo')[bar];
+   ```

UPGRADE-3.0.md

@@ -1624,3 +1624,20 @@ UPGRADE FROM 2.x to 3.0
  * `Process::setStdin()` and `Process::getStdin()` have been removed. Use
    `Process::setInput()` and `Process::getInput()` that works the same way.
  * `Process::setInput()` and `ProcessBuilder::setInput()` do not accept non-scalar types.
+
+### HttpFoundation
+
+ * Removed the feature that allowed finding deep items in `ParameterBag::get()`.
+   This may affect you when getting parameters from the `Request` class:
+
+   Before:
+
+   ```php
+   $request->query->get('foo[bar]', null, true);
+   ```
+
+   After:
+
+   ```php
+   $request->query->get('foo')[bar];
+   ```