Restricted files on El Capitan? So I wanted to hide the Finder icon on my Dock. 
According to this answer - How can I remove the Finder icon from my Dock? I am suppose to edit DockMenus.plist file. When I tried I had no permission to do that. I tried to use chmod (also with sudo) on that file with no effect, same message from terminal - Unable to change file mode on DockMenus.plist: Operation not permitted. I ran ls -lOe on that file and I learned that apparently there is new permission flag called restricted.

I still don't know how to get permission to edit that file. Does anyone have any solution?
I have solved this issue only by booting to Recovery Mode (press CMDR during restart) and launching Terminal. This terminal doesn't care abour restricted flag on files. Also found answer here:
How do I disable System Integrity Protection (SIP) AKA &quot;rootless&quot; on OS X 10.11, El Capitan?