No, DLC's are specific to the platform they were purchased on. XBox and Microsoft Store purchases are only shared if the games have the Play Anywhere logo on them (which is mostly Microsoft produced games). None of the Destiny 2 content have the Play Anywhere log on their store pages in the XBox or Microsoft Store. Thus if you purchase a DLC on the Microsoft Store you will only have access to it when playing the Microsoft Store version of D2 on a PC.
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Perfect, thank you for your detailed answer!