The extension's composer.json reads:
"php": ">=5.5.9"
But the code is already using PHP 7.1 nullable types, as spotted by phan in https://gerrit.wikimedia.org/r/c/mediawiki/extensions/Wikidata.org/+/647228. Perhaps it's time to bump the requirement to PHP 7.2+? Or is these some good reason to keep it at 5.5, and we should remove PHP 7 features?