Quoting [@jrockowitz][1]:

> WebformSubmission data is not stored using Field API (see: [#2792583][2]).

> You would have to query the `webform_submission_data` table and collect the submission ids (sid) or use the *Webform Views* module.

Source: [Entity query webform submission data condition][3].


  [1]: https://www.drupal.org/u/jrockowitz
  [2]: https://www.drupal.org/node/2792583
  [3]: https://www.drupal.org/node/2858727#comment-11975561