WhatsApp iCloud backup stuck at 0% and just won't move
I had this issue on iOS 13 and Whatsapp support magically got back to me.

Whatsapp needs 2.05x the backup size available on your iPhone, and in its infinite wisdom also in iCloud.  So if your backup is past 2.4ish GB, you are essentially forced to start paying Apple for additional iCloud storage.

I can personally verify this worked.  I tried everything (other troubleshooting steps include turning off iCloud Drive and deleting anything Whatsapp already put in iCloud).  After getting stuck randomly between 0%, 4% and 10% I moved to 50GB, flipped back to Whatsapp and it immediately continued progressing.

Maybe one day Whatsapp will let us manually backup to Google Drive or otherwise just spit out a file we can restore from...