Perhaps a bit of stupid thread here, but wondering whether I’m alone on this one 
I’ve been keeping all my board passes in my Apple Wallet. I’m a collector in nature so struggling to delete them, but feel I need to as it makes difficult to use other cards I have in there.
Am I the only one keeping these? Whatks your strategy? 
I keep them until either my trip has finished, or in case I’m owed compensation (EU 261 for a delayed/cancelled flight), until I get that. With some airlines I get my boarding pass very early and having too many cluttering it up gets confusing! (“No, that flight to London is next month, and with another airline, I can’t let you board” - thankfully that was easily fixed!)
I start deleting after some trips - I find that the deleting function is badly developed, or I am blind. Every boarding pass individually?!
I fly on average four times a week this is becoming quickly a large number …
I know! 
need to let it go
On a related but tangential note: My father kept ALL his boarding passes from the time he started flying for – well until he died.
When he retired and started gettign into the newfangled computer thingies, he wasn’t certain what to do with a spreadsheet, but it sure looked like it would be fun to enter all the data from those boarding passes. He had a lot, too – he traveled extensively for work.
He kept a running summary of time spent flying, miles flown, etc. etc. It seemed to keep him occupied.
I wish I had a copy of the file.
So, some day, you might find it interesting to review all the traveling you’ve done.