Why would they need to make a change?? Both A and I are one-up from the lowest levels (G and Z). Nothing has changed there. You will pretty much never see a case where there is G/Z availability, but no A/I fare availability. Don't think you understand how the fare buckets work.
No I understand. I also saw previously where G was available but the search engine wouldn't accept it when Delta went to GAP fares, not just A/P for domestic first class. I believe it may be the same bug here now that Delta has aligned domestic first class fare buckets with business class buckets, leaving the GAP buckets for Premium Select.
I am seeing Z availability for 2 people ATW-ATL on my dates I am interested in, but when doing the search it only returns I for the companion certificate. My only other thought is there is another restriction on the availability of the the Z fare with companion code, though I suspect the search engine isn't looking for Z since the terms didn't have Z in it previously and at the moment still don't. In this case it is about a $190 price difference, not insignificant if still a $535 dollar savings over buying the two tickets outright.