Why did iCloud delete all of my photos saved to it from the past several years when I have plenty of storage available?

It's possible that you device storage was insufficient or inadequate as well. iCloud is not a traditional cloud-based system like Dropbox, Google Drive or Photos, OneDrive, etc. Instead it's a system that syncs your data across all devices, stores that data in the cloud, and also stores a smaller footprint of that data on your device.

For example, folks believe that when they store photos in iCloud Photos, it's only being stored in the cloud but in actuality, it's also being stored on your device which takes up that device's storage space as well. That's why it's advised to utilize the Optimize Phone Storage feature (Settings > name > iCloud > Photos > Optimize Phone Storage) to minimize the storage space used on the device.