Does the Scarlett 18i6 have "loopback"?

No, the 18i6 doesn't have the loopback function due to a limitation of DSP. However there is a workaround for this, using the SPDIF in/out. If you loop a cable from the SPDIF output to the SPDIF input on the 18i6 you can achieve similar functionality as with loopback and with no degradation to the signal since it remains a digital signal.