Cannot delete Outlook calendar event on iPhone (recurring event) - SOLVED!

I have an iPhone and use an Exchange account (Exchange Online in my case) for email, calendars, etc. However I have been finding old recurring events pop up - e.g. recurring meetings from a company I no longer work for. When they pop up reminders, I am only offered the option to "Decline", and no matter what I select after that (this event or all events) it then tells me it was unable to save my "response" and the event remains and keeps popping up.

EDIT: The PROBLEM turns out to be old "invitations" and "acceptances" (and maybe even "declines") hanging around in your Outlook folders. For example unresponded-to meeting invitations get interpreted by the iPhone as "Calendar Events"!

The SOLUTION I've found is getting rid of all the "invites" and "acceptances" of Calendar events using Outlook instead of the iPhone:

(1) Open Outlook on laptop or desktop

(2) Enter something in the Search bar (I started with "Meeting" and eventually went to common letters like just "e" since Outlook won't let you just include all items)

(3) Select "All Outlook Items" at right of search bar

(4) Under results, click to sort by the "icon" column (this may take a good while to finish sorting, btw)

(5) Scroll down to near the bottom of the list. Just above any "undeliverable" items you should find a whole bunch of invitations, calendar items, etc.

(6) Select all the items with a "Calendar icon" in the icon column on the left, and delete them!

• Note it may take a good while for Outlook to do the delete
• Note it may also take a while (for me, overnight) for the deletion to finally sync with your phone
• This procedure can also be done online on Outlook.com if you don't have access to Windows Outlook app, but their search is clumsier (doesn't allow sort by icon column) so you'll have to search globally for individual names of events/invites/acceptances and then manually delete them.

Would have posted this in response to one of the many hapless threads about this (that I found when I was having this problem yesterday), but Apple has an ill-advised "can't respond to old threads" policy. Since I never found the solution in one of their existing threads but eventually figured it out for myself, I'm posting here where hopefully someone will find it "for posterity".