iPhone 6S Plus - Macbook Pro USB Charging not reaching 2.1A
I have the same 2015 15" MBP, just upgraded to OS 10.13.4, and with my iPhone 6S+ discharged to 13%, I got: 


Current Available (mA): 500
Current Required (mA): 500
Extra Operating Current (mA): 1600
Sleep Current (mA): 2100


BUT, I only get that in the right USB port. When I plug it into the left USB port at 13% charge, it Beeps once a second, so I removed it. Once the phone is charged to 20%, it will charge in either port. The phone is now charged to 98%, but the refreshed System Info still shows the same values.