How can I delete photos from my iPhone to free up storage but still keep them stored in the cloud?

There are many ways of preserving photos and videos.

• emailed, texted or messaged the photos to friends or family
• iTunes backup
• Transfer your photos to your computer via USB cable
• Sync to Google Photos
• Sync to some other photo archiving service such as Shutterfly
• Sync to Dropbox
• Sync to box.com
• Sync to Microsoft OneDrive
• Sync to Amazon Photos
• back up to Livedrive
• Backup to any other backup or archiving service
• Transfer them to a Flash drive through the lightning port on your iPhone, using a product such as SanDisk 64 GB iXpand Flash Drive Go - Apple

You can’t. iCloud Photos is not off device storage. It is a syncing service. What you do on one device is reflected everywhere including in iCloud.

Set up and use iCloud Photos - Apple Support

Photos delete from all of your devices

When you delete photos and videos on one device, they're deleted everywhere that you use iCloud Photos. If you delete something accidentally, you can recovery it in the Recently Deleted folder for 30 days. After that, it's deleted permanently. 

It is much safer to store your photos on your computer or an external disc. Many have learned that lesson the hard way....

You can also enable Optimize Storage; this saves reduced resolution copies of your photos on your phone, but keeps the full resolution in iCloud.

But doesn't Apple sync with your folders and then everything is gone?

iCloud syncs with the Photos app on a Mac, and with iCloud for Windows on a PC. If you download the photos to folders on your computer then they won’t sync with iCloud.

My post was in error: I was on the wrong thread.