I currently use google but am thinking seriously about self hosting. For those few times could the clients send it to you via WhatsApp?
Like, sure, you could probably get away with it. Once at most places. And even then, it would still likely leave a sour taste in your client’s mouth if their new vendor couldn’t even get their shit together to keep their email up and running smoothly.
You had a mid-sized to corporate business sending passports over WhatsApp? I highly doubt that.
This is not even coming from a “US-centric” view. This is coming from a “that will never pass any sort of compliance or hell corporate governance” sniff test.
Before the Russian/Ukraine war, I worked for a US company that did business operations on Russian cloud providers - one (very major one I don't want to name here publicly) had their main support in some sketchy all-russian telegram channel that I only could access via my personal device. You'd type your issue in english, get a bunch of russian responses, and then sometimes hours/days later your issue would mysteriously vanish.
"I don't know, dude is usually black out drunk I will go check on him"
5 min later
"Yeah he was passed out on the floor, I threw a bucket of water on him and rebooted the thing it should come back up shortly."
...and yet WhatsApp Business exists to do exactly that. In developing countries, some businesses use WhatsApp as the only written-communication channel (supplemented via voice calls). For over a billion people, their smartphone is the primary - oftentimes only - computing platform. Business communication over WhatsApp is not as awkward as you suppose it is, and all businesses, including large ones, have to meet their clients on whatever platform(s) they happen to be on.
To answer your question, no I've never corresponded with a client via WhatsApp