Sherry Chen, since you gave the answer below on May 7, numerous people have asked meaningful follow-up questions, as well as noting they can't find ANY data, let alone migrated data in the "On My Computer" folder.
The notice we all received is not detailed enough to merit me hitting DELETE and just trusting I'll have all my data.
Please advise; thank you.
Dear LK Hill,
Good day! Thank you for posting to Microsoft Community. We are happy to assist you.
Yes, this is a legitimate message from Outlook. When you migrate data from Legacy Outlook to New Outlook, a copy of the data is stored on your computer. The message you are seeing is a reminder to delete the copy of the migrated data to free up space on your computer. The migrated data is still available in New Outlook and can be accessed in the "On My Computer" folder.
If you have any further questions or concerns, please let me know.
Thanks for your precious time. Have a nice day.
Sincerely,Sherry | Microsoft Community Moderator