Q: Deleting iCloud backups from old iPhones

You can delete a backup that is on iCloud for a phone that you are no longer using without it affecting your current phone. Go to Settings > iCloud > Storage > Manage Storage and tap the old device name in the Backups section and then tap Delete Backup.

There are other things that can be done to reduce the storage you are using in iCloud discussed in Manage your iCloud storage - Apple Support