Currently, Outlook JavaScript API only supports requirement set 1.5 and 5 additional APIs from newer requirement sets, all around signature insertion (see https://learn.microsoft.com/en-us/office/dev/add-ins/outlook/outlook-mobile-apis#supported-apis).
As in many cases solutions are implemented on both the client and server side, having the possibility to set a header to the email to mark the message as processed/touched by the Web Add-in is a requirement so that the server-side solution does not insert the signature again.
Add support for the
Office.context.mailbox.item.internetHeaders.setAsync() and
Office.context.mailbox.item.internetHeaders.getAsync()
APIs from the requirement set 1.8.