Is it normal that External Hard Drives continue to be listed in Disk Utility / diskutil even after Eject?
This is completely normal and no issue to worry about.

When you unmount a volume you are just removing the read and write access to it but the computer knows that the HD is still conn cited to your computer you can always mount it again by clicking disk utility and click the volume and mount it to make the HD disappear from the list you have to physically remove it.