Hi!
Due to upgrade to more current version of Sesame toolkit, the format of JSON output of Wikidata Query Service has changed slightly[1]. The change is that plain literals (ones that do not have explicit data type, like "string" or "string"@de) now have "datatype" field. The language literals will have type http://www.w3.org/1999/02/22-rdf-syntax-ns#langString and the non-language ones http://www.w3.org/2001/XMLSchema#string. This is in accordance with RDF 1.1 standard [2], where all literals have data type (even though for these types it is implicit).
I apologize for not noting this in advance - though I knew this change in the standard happened, I did not foresee it will also carry over to the JSON output format. I am not sure yet which output form is actually correct, since standards seem to be conflicting, maybe due to the fact that JSON results standard hasn't been updated since 2013 and RDF 1.1 is from 2014, so I will research which form is more correct. But for now I would recommend to update the tools to recognize that these literals now may have type. If I discover that the standards or accepted practices recommend otherwise, I'll update further. You can also watch https://phabricator.wikimedia.org/T225996 for final resolution of this.
[1] https://phabricator.wikimedia.org/T225996 [2] https://www.w3.org/TR/rdf11-concepts/#section-Graph-Literal