Right. However there's many competing post services providers in my country, and the ones used by the government and institutions change from time to time (there's open competition every few years and for a while courts were using different provider than the usual national post for example). And no matter which one was "mandatory" at any given time - all of them have to keep the secret of correspondence and have to comply with the law regarding storing private informations etc.

ING, a bank, is starting to require WhatsApp instead of email for customer service, in the Netherlands I think.

You're not that far off ;-)

Such a requirement would terminate my relationship with them immediately...

What if every bank started requiring it?