When Keeper.sh removes one of its own copies from an Outlook or Microsoft 365 calendar, the copy is now deleted outright. Before this release, each removed copy went to the account's Deleted Items folder, the same as an event you delete by hand.
Copies are removed whenever the original is deleted, moves out of the sync window or stops matching your filters. On a busy calendar, those removals could fill Deleted Items with thousands of old copies.
Only copies Keeper.sh created are deleted this way. Events you delete yourself, and every event in your source calendars, follow Outlook's usual rules.
How a copy follows the original explains when Keeper.sh updates or removes a copy.