Hi,@ComputerHabit
Thanks for posting your question in the Microsoft Q&A forum.
Based on your description,I know that you get error when run Get-EXOMailbox -PropertySets Archive -Filter "AutoExpandingArchiveEnabled -eq 'True'".
I also ran the cmdlet you provided and had the same error. The error message shows that the attribute AutoExpandingArchiveEnabled is not supported when trying to use the $filter parameter.
I checked Microsoft's official document and found that there is no AutoExpandingArchive attribute in the Filter parameters, so it's expected behavior to get this error.
You can refer to this URL for details: Filterable properties for the Filter parameter | Microsoft Learn
If you want to get the mailboxes with AutoExpandingArchive enabled, you can run Get-Mailbox -ResultSize Unlimited | Where-Object {$_.AutoExpandingArchiveEnabled -eq $True} | Select-Object DisplayName,AutoExpandingArchiveEnabled
Since I don't have AutoExpandingArchive turned on for my mailbox, I changed True to False for testing.
If my answer is helpful to you, please mark it as the answer so that other users can refer to it. Thank you for your support and understanding.