Once iCloud storage has saved past data, such as photos, can you have them deleted, to make room for your future data?

iCloud storage is syncing storage. That means that unless you disable the syncing for a certain category that what you store on one device is stored on other devices too. However, if you delete a file on one device it is also deleted everywhere else that uses iCloud for that file.

iCloud is not like a traditional archival drive where you can offload items. The exception is optimization but note that this entails lesser quality files on devices. Click on the link to learn how to optimize the storage on a device --> "How to check the storage on your iPhone, iPad, and iPod touch" - How to check the storage on your iPhone and iPad - Apple Support

- "If your device is low on storage, it automatically frees up space while installing an app, updating iOS or iPadOS, downloading music, recording videos, and more.

I don’t want to save the photos to iCloud.

Change your iCloud feature settings [iOS and Mac] - Change which apps sync and store data with iCloud - Apple Support

You can turn off iCloud syncing for Photos. The photos will then only be on your computer (except the ones that were already in iCloud, in which case log into iCloud.com and delete them from there so they are not in iCloud). Make sure you use another way for backing up your photos such as to a computer.