I have seen reports that the Delta website also directs passengers to Sherpa.
I was curious about that, John, so I checked and you're right. If you go to the destinations advisory page, there is a large blurb about the UK ETA, along with a link to "apply online" that takes you directly to the Sherpa website. It's not like one of the scammy sites that charges a fee of $100 or more, but it does charge a $9 fee for applying on the Sherpa site.
FWIW, Delta is not the only airline that is using Sherpa for visa info and applications. Others include AA, KLM, British Airways, Air Canada and many more. You can find the whole list here (scroll to bottom): https://apply.joinsherpa.com/travel-restrictions