OmniFocus has long had the ability to drop projects that are no longer part of your system. Dropping is like deleting, except that the item stays around in the database, should you ever want to find it again. More likely, you may just want to keep a record of not only what you chose to do, but things you chose not to do.
Starting in OmniFocus 3.4 for Mac and 3.3 for iOS, actions will gain the same ability to be dropped. You can sign up to help test the feature.