WhatsApp (now, since it moved into FB's infra) connects to g.whatsapp.net, which is a CNAME to chat.cdn.whatsapp.net, which in turn is an A record to a VIP on Facebook's edge network. The A record you're returned can change — it's intended to be one that's closest to you (as determined by your DNS resolver's location and probably EDNS Client Subnet) but traffic engineering policies might cause different responses over time.

Since on a flight it's likely everyone will use the same resolver on the ground somewhere in Delta's infrastructure a simple mechanism to resolve the IP periodically and update a whitelist (or to cache one VIP location and always return that) might work. Alternatively, as other commenters have suggested, it'd be better to identify traffic with SNI or other profiling.

Updates to the app almost certainly won't change the address it connects to.

> Updates to the app almost certainly won't change the address it connects to.

Well, they have before. There's four generations of hostnames that were used before g. But g seems likely to work for quite a while.