Why does Trezor retain customer data from December 2021, and what is their need for this data, since the user does not interact with the company directly after purchasing the devices?
It's not Trezor's data. It belongs to the 3rd-party service they use for the customer support portal. Their TOS and Privacy Policy will shed more light on how long they retain customer information.
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To my knowledge Trezor only holds onto customer ordering info for up to 30 or 90 days I can't remember which. I am curious to see what happens to their reputation if anything as a result of this hack. It appears that it was largely outside of their control seeing as one of their third party vendors was hacked and they were not directly hacked.