Updated on July 14th, 2019. The Sketchfab filter plugin for the Moodle LMS has not been compatible with newer versions of Moodle. But this should not prevent you from taking advantage of Sketchfab. In fact, the new Sketchfab viewer makes it available for any HTML editor. Just ask Osokorn II:
The Sketchfab for Atto filter plugin for Moodle allows you to add a Sketchfab model to your Moodle site. When installed, a toolbar button click will allow you to paste the Sketchfab model URL onto a field, and the model will show up after you save the editor.
Available for Moodle 2.7 to 2.9. Download Sketchfab for Atto filter plugin from the official Moodle plugin directory.