OSX Photos Events and Albums Have same photos
Events are now just albums. So, you can remove specific photos from Albums or "Events" without deleting the photo. Only if you delete a photo from the main Photos view is it actually deleted (and then, under File: Show Recently Deleted, you can recover it for 30 days).

I recommend deleting events that are no longer useful, as the Photos view now nicely converts into years and locations as you zoom out, which you might previously have manually set up as events.