The documentation says:
On-send functionality is only supported for user mailboxes in Outlook on the web, Windows, and Mac. The functionality is not currently supported for the following mailbox types and modes.
- Shared mailboxes
We see that on-send does work with shared mailboxes. Has something changed recently to make the docs inaccurate?
ItemSend should work with the SupportsSharedFolders tag: https://learn.microsoft.com/en-us/office/dev/add-ins/reference/manifest/supportssharedfolders
But it should not work without that feature. The Documentation should be updated to be more clear.