Still shows up on Garmin Connect? Might be because you have rides uploaded to the site that used that device. Or because GC simply doesn't do that.
I've never worried about removing the devices I no longer use. Long before Garmin Connect, there was and I think there still is a myGarmin site that you registered your devices too and it helped you track updates and other stuff. If you gave your device to another and they wanted to do map updates, then you'd have to release your claim on the device through that site.
Don't know how they do it now, I know or at least think. that at one time Garmin Express was automatically putting any new device you added through GE to the myGarmin site as well as Garmin Connect.
Hmmm, just took a look at my mygarmin.com. After a few redirects it took me to a page and I can see everything I ever had and more. Pretty much anything I ever updated with Garmin Express whether actually mine or not.