In my case, all of the 'phantom' objects are for Events that I've declined but were sent to a list address OR Events that I was listed as the "organizer" for but were NOT created in Calendar.app (either crated in OWA or MS Teams).
So I did some additional checking. ALL of the Events in my exchange account that show the "This event was imported (read-only)" message have the following things in common.
1. It does not matter whether the organizer shows as me or not
2. It does not matter whether I am in the 'To' list or received the invite as part of a list.
3. For events where I AM the Organizer, the Event was NOT created via Calendar.app.
And then did a BUNCH of testing to see what would / would not result in a 'phantom' Events that could not be deleted.
I added the mailing list email address for several 'phantom' Events to my contact card. I closed all apps after saving to Contact, verified the updated details showed up in my contact card on iCloud.com (I don't have any contacts in my exchange account and I don't think Exchange has a concept of a "me" contact card anyway), and then rebooted. I still see the events and cannot delete them.
I also noticed that I could NOT delete events where I WAS in the TO list OR where I was the organizer and the event was created outside Calendar.app (M$ Teams or OWA).
I also tested whether having an "online meeting" changed anything - it did not.
Additionally, I found that cancelling Events that I had scheduled outside of Calendar.app resulted in 'phantom' Events that still could not be deleted - even though they no longer show up in OWA, on mobile (iPhone / iPad) or M$ Teams.
Finally, for meetings that were addressed directly to me, I no longer have an option to change my response option from 'accepted' to 'decline' or 'tentative' (maybe) either. So there is no way to REMOVE ANY Events in Calendar.app any longer.
Guess I'll be using OWA and Teams for everything now.. Might as well stop using Mail.app as well since I'll have to be in OWA anyway.