[lnl]

Sometimes it's necessary, when the protocol itself is updated, e.g. the multi-device beta page currently says "Messaging or calling someone who is using a very old version of WhatsApp on their phone won't work from your linked device."[1] for this reason. If they don't force those users to upgrade at some point, it would lead to an undesirable situation where you are not guaranteed to be able to message someone from desktop. My experience with my grandmother's phone confirms gnicholas's account that they do this very rarely.

But, more importantly for gnicholas's point, Whatsapp gives you a warning days/weeks ahead about the need to update the app. My grandmother is almost never connected to a Wi-Fi network, so her apps don't usually update, and she hardly uses any app other than Whatsapp anyway. She got the warning one time and gave me a call immediately, so I walked her through about what to do (i.e. yes grandma, click the update button). If it silently fails, like gnicholas says, and if this happens often, then I can't imagine using Signal with her.

[1]: https://faq.whatsapp.com/general/download-and-installation/a...