There's things I don't like about that approach, but I do like that it works with WhatsApp web.
Primarily the annoying thing about it is that it opens in a new window (like our other share buttons do) but then that window stays open. Or at least it does if you are using the desktop client. There's not a lot we can do to change that so we'd just have to put up with it.
I'm not 100% sure what the behaviour is with WhatsApp web. Presumably you have to click "Send". Does that open another new tab?
I'm going to change it and roll it out here, but I'm a bit torn. The other thing the api link helps with is that it will presumably work whether or not you have a WhatsApp client or even WhatsApp account which will avoid the perhaps unexpected behaviour of the button just not doing anything in these cases.
Let's see how we get on... (should be live now).