Terminal: Operation not permitted

Possibly there are older system files that should but could not be deleted. Error message "Operation not permitted".
Follow these steps:

1. restart and open recovery mode with <cmd>+<r>

2. login with one account offered

3. open terminal under disk utilities

4. disable SIP

   csrutil disable

5. restart

6. delete file/folder

7. restart again in recovery mode

8. login again

9. open terminal

10. enable SIP

    csrutil enable

11. restart