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...
And yes, I do have Signal installed, and there are only 2 people who talk to me through it (one being my partner).
Phone carriers got too greedy charging for every single SMS message and phone call, WhatsApp took over when smartphones became popular.