Can you back up your iPhone then delete content on iPhone and it will still stay on iCloud ?

If you backup your iPhone to iCloud, you get a closed package form which you cannot extract content. You can only restore the entire backup to the iPhone, by erasing the iPhone and setting it up again. If you delete content from the iPhone after making a backup, the backup will be unaffected, but you will have no access to the backed up data unless you restore the entire backup to the iPhone.

If you sync content to iCloud (different from a backup), then anything you delete from the iPhone will be removed from iCloud as well, as it is synced.

If you backup the phone then erase your phone your backup ill remain in the cloud. If you erase the phone then just sign in without restoring then certain parts of your iCloud will show up automatically(Contacts, calnedars, notes and photos if they're appropriately turned on). It all depends on what you're trying to accomplish but the majority of your info should be readily available through iCloud.

Also beware that iCloud backups are incremental. If you backup your device to iCloud, delete material, and then the next day's automatic backup is created. That next incremental backup replaces what was in the previous backup with the current contents of the device.

You can make an iTunes backup and then archive that backup, which you cannot do in iCloud. But since restoring a backup is an all or nothing affair, the only way, at any point to see the contents of that archived backup will be to over-write everything on the phone with the contents of the backup.

If you really need to archive data or files, you need to export them somewhere, and in a format that you'll be able to read them in the future. Then delete those data.

Note that with iOS 11 you can also offload unused apps while keeping their data intact on your device. if you need access to that data, just reinstall the app from the App Store. That way you free up space by removing the app but don't lose any data for when you next do need that app.

Settings -> General -> iPhone Storage -> Offload unused apps