Dear [removed]
Good day! Thank you for posting to the Microsoft Community. We are happy to assist you.
Unfortunately, The primary issue with using conditional formatting to track who has read an email in a shared mailbox is that Outlook doesn't inherently store this information in a way that can be easily accessed for conditional formatting. We understand your concern about this feature and how it helps you improve your efficiency in a work schedule. Still, as it is related to Outlook product design, submitting your valuable feedback directly to the Microsoft product development team is strongly recommended by either going to Help from your dedicated App > submit Feedback or posting on Outlook · Community (microsoft.com) for feature requests.
Please note that nobody here is the product developer/decision maker and has no control over the product. Everyone, including us as community moderators, can only assist you with the query for providing available solutions and the feature's availability status within our scope and capability.
To check more details about how to submit feedback, kindly refer to this article Learn about how to provide feedback to Microsoft - Microsoft 365 admin | Microsoft Learn
Thank you for your time and understanding. For other questions, feel free to add your post to the Microsoft Community team.