There's a Sailfish help page [0] showing how to get the APK from Aptoide, or downloading directly from Whatsapp.com .
But with Google killing off 'sideloading', is it credible that independent APK sources are going to dry up in future?
[0] https://docs.sailfishos.org/Support/Help_Articles/Whatsapp_S...
Of course, this doesn't mean that the downloaded app will work on such a device (if it doesn't have Google Play Services), but at least it lets you download the app, which isn't much different from downloading it from say, APK Mirror. And as long as you can extract the apps from either the Play Store or Android devices itself (via adb/root etc), I'm assuming sites like APK Mirror will continue to exist.