You do see a warning when iCloud is low on storage. It's actually given pretty early on.
You said your photos were deleted without your permission, but if you were not using an upgraded version of iCloud then iCloud wasn't really involved in this. You can store a fair number of photos on a device. It will try to copy them to iCloud too, but if you fill up your free iCloud it will stop copying them. You could possibly store all 1000 photos in 5 GB of iCloud space (I never have more than a dozen on my phone before copying them off).
I suspect that we're missing a lot of background to fully understand what is going on here. It may be these photos never were in Icloud, but you must have done something on your device to cause 1000+ photos to disappear. iCloud is only involved to the extent that they should then have reappeared if you had had sufficient storage space in iCloud.