Hi @Ian K ,
Welcome to our forum!
As I know, this feature is only supported in Outlook 365 with Version 1804 (Build 9226.2114) or higher. Based on my test in Outlook 2016(Version 16.0.5197.1000) and Outlook LTSC 2021(version 2108(Build 14332.208281 Click-to-Run)), this option was not found, which is consistent with your issue. It's by design. For more information, please refer to: Show reminders over other programs or apps.
For your requirement, I have found a similar feature request submitted by other user on the website dedicated to collecting feedback. It's suggested that you could vote for this and hope Microsoft will notice this in the future. Of course you can also post your suggestions by yourself on this website, we sincerely welcome your suggestions, and thanks for your understanding and support!
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