WhatsApp Plus, as an application is not legal, as it was created using reverse engineering on WhatsApp Messenger, violating its terms and conditions of use. But its use in itself is not illegal. But you must always be aware of the risks involved.
That said, the use of WhatsApp Plus by its users is not illegal itself since there is no specific legislation to follow, but it does entail a number of risks. The main one of all is that we can be banned from the service "for improper use", that can be indirectly understood as a consequence of using MODs. In 2015 WhatsApp massively banned the accounts of WhatsApp MOD users forcing them to uninstall and install the official version if they wanted to continue using the service. This strict usage policy was relaxed over time giving users some freedom to re-use the MODs without compromising their accounts. However, WhatsApp continues to block MOD user accounts on a regular basis, so you may be affected if you decide to use WhatsApp Plus. And all this without prejudice to other measures that could be taken in the future against the creators of the MODs and their users, such as going from temporary or conditional banning to installing the official app to directly blocking our account associated with our telephone number definitively or opting for legal action.