A new Mark Events as Private setting on a source calendar marks every event Keeper.sh copies from it as private in the destination. People who can see the destination calendar, such as colleagues on a work calendar, see that the time is taken without seeing the event's details.
Keeper.sh uses each provider's own private flag: private visibility on Google Calendar, private sensitivity on Outlook, and the private class on iCloud, Fastmail and other CalDAV calendars. The destination calendar then hides the details the way it does for any private event.
The setting is part of Pro, alongside the other event filters. It suits a personal calendar copied into a work one, where the copies should hold the time and show nothing else.
Keep personal details out of your work calendar shows a full setup.