If you kill the Ender Dragon in the end and then remove all the end portal frames from the portal, then relog, you will see a new bossbar for a new Ender Dragon. You can't target it with @e as it doesn't actually exist yet, but it will spawn when you move far enough from 0 0. Basically this means that a new dragon always spawns if the frames are removed, even if you never placed the end crystals right. This limits what creative players can do in the end, as there's always a portal located (and a dragon, if the portal got removed)