The first argument, $format is at conflict with the second, $stringEncode.

This leads to scenarios where you have code like

$validators->rules('json', false);

or

$validators->rules('json', true);

which makes it very difficult to predict what the output will be. In fact, this is happening within UF Core right now, where the latter example is used.

There is no way this problem could be addressed without a semver major change, as any action would result in potentially changed outputs.