Using WhatsApp Plus isn’t illegal but the development does go against certain copyright laws since WhatsApp has never publicly released its API to be used by other developers to create projects of this nature. However, it’s also true that WhatsApp doesn’t seem to be too bothered by the presence of the Plus mod because, otherwise, they would have put more effort into blocking its use or banning those users who had downloaded the app and were using it instead of the original official application (at WhatsApp Inc. they are perfectly aware of who uses the Plus version and who doesn’t).
It’s also true that some years ago, a bunch of WhatsApp+ users were kicked out of the service but they didn’t receive any further notice other than one about an inappropriate use of the app, which also goes for users of the official WhatsApp that don’t use it properly. In other words, nobody has been officially blocked or banned for using the Plus version.
As we mentioned above, WhatsApp Inc. hasn’t released the API for other developers. Therefore, it’s always hard to say whether projects of this nature are legal or not, and it’s usually the fear of users to being blocked or banned from the service that leads people to think that making use of this app could be unlawful. That doesn’t happen in other chat and instant messaging applications such as Telegram that offers its source code freely so that any coder can develop his own applications based on the original software.