Q: How do you Erase free space only on a USB drive in disk utility El Capitain version 10.11.3?

You can see more detailed web discussions on using erase disk.  Basically the feature was likely removed because it does not really work with SSD drives that are common in newer computers and Apple did not want people lulled into a false sense of security.

Disk Utility (El Capitan): Erase a volume - https://support.apple.com/kb/PH22241 -

https://discussions.apple.com/thread/7376304

How to securely delete files in OS X 10.11 ‘El Capitan’ - http://www.macissues.com/2015/10/03/how-to-securely-delete-files-in-os-x-10-11-e l-capitan/

http://lifehacker.com/how-to-securely-erase-a-solid-state-drive-on-mac-os-x-1580 603733 - Erasing SSD not easily done vs. HDD so Apple removed it in 10.11 so people wouldn't have false sense of security

http://www.macworld.com/article/3005796/operating-systems/how-to-replace-secure- empty-trash-in-os-x-el-capitan.html