The RTE in EE6 is built on CKeditor 5 which dropped the view source option . This has a caused a lot of controversy in the CK editor world as you can see on this thread https://github.com/ckeditor/ckeditor5/issues/592 so its not an option for the RTE editor. More information here too https://expressionengine.com/forums/topic/252969/what-happened-to-the-view-code-button-in-the-rte
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