IPhone – WhatsApp won’t download on iPhone 7

No, iOS can't delete your WhatsApp account. iOS can't, on its own, delete any account for any app.

You stated you didn't have Offload Unused App enabled so it's more probable that WhatsApp didn't have the available space it needed to store your history.

iOS will determine if it needs to 'unload' an app from memory to make space for system process, another app's use, etc. I can't say for sure but it appears that WhatsApp was removed from memory because the system needed to reclaim that memory to use for something else.

If you delete apps you don't need, turn on Offload Unused Apps, and then reinstall WhatsApp, you might see your history return. This answer from a similar question might help, too. That user stopped all apps using the app switcher and then restarted their iPhone. That helped them regain space and add their apps back.

The important thing is that you restore the backup on the iPhone 6. Once restored, it disables the backup of that iPhone 6 so that it does not replace the original copy of the iPhone X. Then when you have your iPhone X again, you will be able to restore the copy original.

On the other hand I recommend that you make a backup in iCloud of all the content and a backup in iTunes.