If a Looker user deletes a scheduled plan, you can still find evidence of the deletion in the Event Attribute System Activity explore.
You can use a query like this:
https://EXAMPLE.cloud.looker.com/explore/system__activity/event_attribute?fields=event.category,event.name,event_attribute.name,event_attribute.value,event.created_time,user.name&f
Note that the Event Attribute table only stores the most recent 100,000 rows. So, if the scheduled plan was deleted a long time ago, it may not show up in the Event Attribute explore.