Does Icloud delete files without warning?

iCloud does not delete files automatically, but it will stop working reliably, if you have not enough storage. The upload to iCloud my not succeed, also the download from iCloud, until you have freed enough cloud storage. See: Manage your iCloud storage - Apple Support

If you run out of iCloud storage, your device won’t back up to iCloud, new photos and videos won't upload to iCloud Photos, and iCloud Drive, other iCloud apps, and text messages won't stay up to date across your devices. And you can't send or receive emails with your iCloud email address.

You should keep backup copies of items you are storing in iCloud, so you can recover items, that might get lost because of user errors or transmission errors. If you are using "Optimize Storage" and not all items are mirrored locally on your Mac, your regular Time Machine backup cannot include the optimised versions, and you will have no backup at all. If you are using iCloud, it is even more important to make regular backpacks, or you may not be able to access your documents and data at all, if you find yourself without an internet access. See: archive or make copies of the information you store in iCloud