WhatsApp multi-device support, which allows you to send and receive WhatsApp messages on your desktop without being connected to your phone, is now available in the stable version of the iOS app.
Previously it was only available to WhatsApp Testflight users on iOS, which is an extremely limited and mostly closed group of users.
If you have version 18.104.22.168 of the app, you may be offered the prompt to link devices.
You can link up to 4 devices, including a macOS/Windows computer, a WhatsApp Web session, or a Portal, but only one, your main device can be a phone.
Once linked, you can send messages without your phone needing to be connected to the Internet.
The feature may be rolling out gradually to users, so be sure to update your app regularly.