Implemented solution via #3066:

The following rules are affected:

• UnusedAssignment (Best Practices)
• UnusedFormalParameter (Best Practices)
• UnusedLocalVariable (Best Practices)

The following variable names are ignored:

• any name starting with ignored, e.g. ignoredParameter
• any name starting with unused, e.g. unusedVar

Is your feature request related to a problem? Please describe.
UnusedLocalVariable, UnusedFormalParameter flag all unused variables.

Describe the solution you'd like
Please add a property to the rule specifying a pattern such as starts with $, _ or unused for the variable name so that variables that meet the criteria are excluded. Additionally, you could support custom annotations that mark the variables as Unused. e.g., @Unused or @Ignored.
E.g., https://github.com/Lanchon/r8/blob/e22a77d26e30615bfc9b5f598bb0f58f10dd082c/src/test/java/com/android/tools/r8/shaking/ifrule/IfOnAnnotationTestClasses.java

Describe alternatives you've considered
Example code flagged by ForLoopCanBeForEach when usual for loop used:

@SuppressWarnings("PMD.UnusedLocalVariable")
  @Override
  public Solution solve() {
    int[] dp = new int[capacity + 1];

    List<Item> itemsList = new ArrayList<>(items.length);
    int i = 0;
    for (int dpVal : dp) {
      for (Item item : items) {
        if (item.weight <= i) {
          int includedVal = dp[i - item.weight] + item.value;
          if (includedVal > dp[i]) dp[i] = includedVal;
        }
      }
      ++i;
    }
    path(items, capacity, dp, itemsList);
    itemsList = Item.pack(itemsList);
    return new Solution(itemsList, dp[capacity]);
  }

Additional context
Haskell/Python style
Error-prone

Also see #2923

Referenced from: #2838