The "Send an email" action allowed mails to be sent using send as permissions. In my case, I've used this to collect form responses and forward these by email. Using the send an email action with the send from field configured to send an email as the form responder.
This provided two major benefits:
1) control over who can email using a certain form, as the admin account used to configure the form needed to have send as permissions on the mailbox of the form responder. This is a manual operation, but allows very strict control.
2) form submissions processed with the send an email action are delivered in my inbox with the proper sender identification, rather than a generic admin account.
Recently, this action was updated with a newer version (send an email (v2)), in which the send from field turns out to have been deprecated. This has been confirmed by Microsoft support in a case I had raised with them.
See attached screenshots to clarify.
Current work around is to capture the responders' email address in the body of the email created, but this still required manually checking who actually send the form.
My suggestion would be to bring back the send from field in the send an email action.