Why would anyone want the invitation email to be deleted?!
90% of the time the email has other information and to check that I either have to go into the calendar and look for the event and click on show the mail to have a second look (and if the event is in one week or two good luck finding that), or I have to go into deleted messages and undelete the email.
The option to disable that only exists in the web version but that does not change it for Outlook on Mac and iOS. I even tried to come up with a rule to get back messages with invitations from the deleted folder but you cannot apply rules on deleted messages (contrary to other mailing services).
Now I duplicate all invitation emails and I RSVP to one of them to keep the other in my inbox, but this is a very messy solution to my taste.
It's really annoying and I would like to keep using the Outlook app without switching to Apple Mail to keep work-related stuff separate from my personal ones but this is a huge problem that needs fixing ASAP.