This line will skip validation of any configuration data for the 'deprecating key' -- the new key.

If someone specifies :oldkey and :newkey in the input data, then the data for :newkey will not be checked - so if bad data is specified it will be passed thru successfully.

It may be better here to do an explicit check for data.key?(:newkey) and add an error that the user has specified both an old and new format.