Hello,
UPDATE: the strykethrough style was included in DOCXCustomizer in phpdocx 12.
The strikethrough style is not one of the supported styles in DOCXCustomizer in the current version of phpdocx. On https://www.phpdocx.com/api-documentation/docxcustomizer/customize-docx-Word-documents-PHP you can read the supported styles by DOCXCustomizer.
We move the request to the dev team to add support to that style with DOCXCustomizer to the next release of phpdocx.
Using the current version of phpdocx, DOCXPath can be used to add new tags to elements. insertWordFragment with a WordFragment generated with addWordML could be used to insert w:strike tags.
Regards.