While saving a page:
Class 'google\appengine\api\taskqueue\PushTask' not found
Maybe it just needs the composer dependency google/appengine-php-sdk.
Change 437291 had a related patch set uploaded (by Hashar; owner: Hashar):
[mediawiki/extensions/GoogleAppEngine@master] composer require google/appengine-php-sdk + test
Change 437291 abandoned by Hashar:
composer require google/appengine-php-sdk test
Per T274069: Archive the GoogleAppEngine extension.