This is done for performance reasons. Quoting the pull request
introducing this option [1]:
> A config might be overkill, but I wanted to provide an escape hatch
> for any upgraded apps that might be testing the exact value of the
> action_dispatch.parameter_filter header.
Since we don't test the exact value of action_dispatch.parameter_filter,
we can enable this option.
[1] Pull request 46452 in https://github.com/rails/rails