Transfer WhatsApp by copying the internal app files

As of 2025, the default method of WhatsApp to Backup and Restore requires a Google Account and Chat data transfer using local Wifi requires Android features that are not available on Sailfish OS.

You will need to have the developer access enabled (access via SSH) and system administration experience is of good value.

Turn off Android: As a first step, open the Settings App, go to System, and select Android™ AppSupport. Tap the Stop button and wait until he action is completed.
Open Terminal or Login using SSH
Enter devel-su
Run tar -czf /home/nemo/whatsapp.tgz /home/.android/data/data/com.whatsapp/
Transfer the resulting file to the new phone

Start WhatsApp once and stop it again
Open the Settings App, go to Apps, select WhatsApp and delete the data and clear the cache.
The, still in the settings switch to System, and select Android™ AppSupport. Tap the Stop button and wait until he action is completed.
Open the terminal and enter devel-su
Run ls -ld /home/.android/data/data/com.whatsapp/ and remember the owner and group ids. They both are the same and likely look like 100XX.
Delete the existing directory rm -r /home/.android/data/data/com.whatsapp/
Run cd / && tar -xzf /home/nemo/whatsapp.tgz
Reset the permissions as read before: chown -R 100XX:100XX /home/.android/data/data/com.whatsapp/

Now start WhatsApp and enjoy!