As the official WhatsApp site has just informed:
The multi-device beta is an opt-in program that gives you early access to try a new version of WhatsApp for Web, Desktop, and Portal. If you join the multi-device beta, you'll be able to use linked companion devices without the need to keep your phone connected.
The option to connect to beta is now enabled for all accounts.
To start using WhatsApp multi-device beta, please follow this guide.