It is not possible to set the autocomplete option to an unchecked state. If autocomplete is disabled via Group Policy, this option is grayed out, but it remains in the previous setting state after canceling.
It is recommended that you ask users to manually turn off this option for each user when they use Outlook.
Also, I have found a similar post in feedback collection website, and you could vote for it. Many features of our current products are designed and upgraded based on customers’ feedback. You can also make suggestions on that website and hope Microsoft will notice this in the future. Thank you for your understanding and support.
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