Using New Outlook on Windows for personal email, calendar, and contact management
Nothing has been officially announced but I expect them to have full feature parity with Classic Desktop when it comes to this feature as it is an important mail triage feature for many people. Also "under the hood" it works similar to custom times and dates for Calendar Items and Tasks.
What you can actually do now as a workaround, and without switching back to Classic Outlook, is to flag the email in New Outlook, then go to the Flagged Emails section in To Do (either via the App or the Navigation on the left in New Outlook), and then set your custom Due Date and Reminder Date/Time for the item.