After you run a migration batch from the ECP, you can view the results and download a detailed report for a specific user. The URL points to https://owaurl/ecp/Migration/ViewMigrationBatchUserDetails.aspx?isNarrow=t&id=%useremailadd%#
If a batch is deleted, does that information still exist somewhere? I tried changing the user in the URL but it says the batch can't be found, obviously. Wondering if that aspx page pulls info from a bunch of logs or if I can dig around and find the info for a user from the deleted batch. Or is there a shell command that I can get migration stats for a specific user after the fact?