Quite often we receive requests from our clients asking how to disable the proofreading option in the source mode of a rich text editor and keep it enabled only in the WYSIWYG mode. In general, for such cases, we offer a special option disableAutoSearchIn of WProofreader that allows disabling it for certain cases (class, id, data attribute name and HTML elements). 

Disabling WProofreader in Source Mode of CKEditor 4

The source mode of CKEditor 4 has a special CSS class '.cke_source'. You can rely on this class and disable WProofreader using the disableAutoSearchIn option with this class specified as in the example below.

<script>
	window.WEBSPELLCHECKER_CONFIG = {
   		....
   		disableAutoSearchIn: ['.cke_source']
  		....
	};
</script>

[Resolved]

WProofreader was disabled by default for CKEditor 4 source mode in release 5.5.2 effective date August 16, 2019.