Hi,
In Exchange Online, there currently is not a direct PowerShell command or mail flow rule that automatically saves emails to a shared mailbox folder when a user completes a deletion operation. Mail flow rules are primarily used for managing the behavior of emails during routing and transmission, not for actions completed by users, such as deletions. If your goal is to ensure emails are backed up before deletion, consider setting up a retention policy. By establishing a retention policy, you can ensure that even if users delete emails, these emails can still be recovered from the “Recovered Items” folder for a certain period. This is not a direct copy to a shared mailbox but can prevent permanent loss of emails.