just a tip....I found the following. Hope it helps:
https://forms.office.com/pages/delegatedpage.aspx?originalowner=<Useridemail>
Example
https://forms.office.com/pages/delegatedpage.aspx?originalowner=jdoe@whateverdotcom
I also noticed this same address....at the top of this article so apologies if its repeated. I tried it and it work for me as I knew the users email who created the from. Just remember the 30 deletion rule (I think that applies also for office 365 users converted to shared mailboxes)
NOTE!:
The user either has to be a shared mailbox or a deleted user in order for that to work otherwise you get a n error stating the user is still a valid account. Of course this solution will only work if you know the users email address that owned the forms. I hope it helps in some small way;