From version 17 onward we may choose to not delete certain types of activities after they are marked as done. This opens up the possibility to report on our employee's performances, client's history, etc.

To configure this we have to access the "Activity Types" menu and after selecting the desired activity click on the "Keep Done" boolean.

This will mark as archived done activities instead of deleting the record. We may find this information from (debug mode active) Settings > Technical > Discuss > Activities. To visualize them we just have to filter by "Active is not set"

This same filter can be used in other apps to signify records that have had an activity done to them: