To enable strikethrough in RTE with the desired icon you want, firstly look for any text file related to RTE. with .js as extension. I've used this file "InsertTextRTEPlugin" ; next step is to save as the rule in your RS and copy-paste the attached code provided in the notepad.
Change the "webwb/StrikeThrough.png", to a desired .png file available in your system, save the js file, and then you'll get to see the SrikeThrough icon with strike through functionality.
I've also attached the basic strikethrough png icon.
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