Quote from the closed bug JRACLOUD-76758 – Deleting a user from the site will remove dashboards they created:

I am now the owner of 97 dashboards rescued from the trash after their creator unexpectedly parted ways with my employer.

Automatic dashboard deletion–intended or not–is a real problem.

I don't know what the ideal solution to that problem is, but "wait for users to notice their dashboards are gone, and then send your admins to dig through the trash like panicked vultures" isn't it.

Note: When a dashboard is moved to trash in the UI it will be kept there for 60 days before being permanently deleted. During these 60 days it is possible to restore the dashboard. For more information refer to the article How to restore a delete dashboard