Last tested: March 2021
By default, in an open system, users have access to everyone's personal folder. If you want to hide those from users, you have two options:
Enable Closed System and then disable Closed System. Enabling Closed System will remove the "All Users" group from the instance. When you disable Closed System the "All Users" group will be recreated, but it will not be added back to each user's personal folder access settings. This will still hide all personal folder, but at the same time give all users access to the shared folder. Disclaimer: New users' folders will be viewable by "All Users", so you will have to repeat this process every time you get a new user.
Go to each personal folder, click "manage access" under the gear menu, and remove "view" permission from the "All Users" group. (this may be much less of a manual effort if the customer is able to use the Looker API as described here)
There is a feature request for a cleaner way to handle this in-product without the need to enable Closed System or the clunkiness of option 2.
This content is subject to limited support.