The classic Outlook has the option "When replying to a message that is not in the Inbox, save the reply in the same folder". As you knоw, the new Outlook has not got this feature.
KeepRepliesInFolder allows the new Outlook or OWA to behave the same way as the classic Outlook with the checked option "When replying to a message that is not in the Inbox, save the reply in the same folder".

Download KeepRepliesInFolder.zip file.
Visit product web page.