Update 4/15/21: It looks like this issue might only be on certain routes such as LAX-JFK and SFO-JFK. For example, Greg was able to find availability from SAN-JFK:
Delta has been making a number of improvements recently from earning status on award flights to extending expiration dates of some Delta credit card benefits through June 30, 2022 to being able to redeem upgrade certificates on award and Miles + Cash flights and more.
With all those genuine enhancements comes a potential devaluation in the usefulness of the companion certificates that come with Delta Reserve cards.
Renés Points has noticed that it seems to no longer be possible to redeem the Reserve card’s companion certificate for Delta One flights. When looking up flights, Chris noticed that only Main Cabin and Comfort+ seats were bookable with the certificate, but when deselecting the certificate there was plenty of availability in Delta One with the I fare class which, in theory, is meant to be eligible.
Chris notes in that post that he’s been able to redeem the companion certificates for Delta One flights in the past, so it certainly looks like this is a devaluation of sorts. However, this might just be Delta fixing a glitch in their booking system rather than a change in policy per se.
The reason why is that Delta’s website states that Delta Reserve companion certificates can be redeemed for the following:
As a Delta SkyMiles Reserve Amex Card Member, you’ll receive a domestic First Class, Delta Comfort+ or Main Cabin Companion Certificate each year after renewal.
Note that Delta One isn’t listed in there and is a different product to First Class. When taking a look on the Wayback Machine to see if Delta One was listed in the past, the earliest copy of that page I could find was from September 21, 2020 and that had the same wording. Greg’s post back in 2019 about Delta’s companion certificates didn’t reference Delta One as being an option and Delta One itself was only launched on April 1, 2018.
That makes me wonder if Delta One flights were never meant to be bookable with companion certificates, but that it had previously been possible nonetheless. If so, the fact that it’s no longer bookable is simply the closure of a loophole rather than Delta taking away something that was explicitly allowed in the past. Still, it’ll be disappointing if that’s how you were planning on using your companion certificates, but you can still redeem the certificates for First Class flights.