In the New Outlook, the functionality to add or customize columns is currently not available. Unlike the previous version of Outlook, where you could find the 'Add Columns' option in the 'View' section, this feature has not been implemented in the New Outlook interface.
If you require column customization, a workaround would be to switch back to Classic Outlook, which supports full column customization. You can follow the steps outlined in the Microsoft support article to add or remove columns in your inbox using Outlook Classic.
Your feedback regarding this limitation is valuable, and you may consider submitting your suggestions through the Microsoft 365 Feedback Hub to help improve future updates.