Revision history is not available in customer Network Sandbox since Apr, 14th. 2018.
It is due to the AWS migration. Historical Revision History data is no longer provided.
This is the same behavior during the Instance Cloning Process (Production to Sandbox clones).
As a workaround, the user can use the local source to review history.
- View a Network entity.
- With the entity displayed, click Revision History from the left-side menu.
- Add a suffix to the address box - source=local