Apple did once plan to allow encrypting backups, but dropped the plan when the FBI complained: https://www.bbc.co.uk/news/technology-51207744

I don't think whoknowswhat11's account (that there was a period where backups were encrypted but it was removed because users didn't want it) is accurate.

I dropped my plan to use Cloud Backups when Apple dropped their plan to protect their users. If you want to know it’s safe, either don’t share it with a third party and hang onto that data yourself. Or alternatively, client-side encrypt it to be sure.

I still remember why I had to use iCloud Backup. It was the safest way to restore my data between iPhone upgrade.

I had seen more than a dozen iTunes Backup corruption where the backup is completely useless. And this problem exist even today. And it seems the chances of happening is higher on Windows PC. iCloud isn't without flaws I have seen three iCloud Backup suggesting it is corrupted. But there is often a backup from previous days as iCloud dont overwrite backup. Given the lower chances of happening I had to use iCloud.

I’ve never once used an iCloud Backup or even local backup between iPhone upgrades. I always set it up as a new phone and don’t migrate any old settings across.

I make the odd encrypted local backup just as a recovery point but my RTO is measured in years TBH.