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Note, that the WebSpellChecker Dialog plugin for CKEditor 4 for Drupal 8 was also deprecated. But still, you can use the SCAYT module instead https://www.drupal.org/project/ckeditor_scayt. Also, you can use WProofreader SDK instead. It's not a native module for Drupal, it is a pre-built JavaScript component that can be added to the Drupal pages with the editor (where the spell check is needed).