Is Delta using older Wifi equipment? I was under the impression that the current Wifi standard now prevents deauth attacks.

Given that the whole wi-fi systems on planes are all very old unless the planes have Starlink, I wouldn't assume it's even remotely new.

Note: Not that it wouldn't be possible to have even SOTA wi-fi 7 and still use the same old legacy satellite uplink WAN connection, it's just that I can't imagine what airline would consider that "upgrade" worth taking airplanes out of service for to perform, when it wouldn't do jack to increase the speeds people would get.

Even if there's newer equipment, Protected Management Frames (PMF) compatibility with older laptops (for example) would be problematic. This is also the reason why WPA2/WPA3 co-existence mode fails on older equipment that is compatible with "normal" WPA2 (because PMF is mandatory in WPA2/3 coex mode).