ED-E is a companion, so he can still die in hardcoe mode like other companions. That's not a bug.
However there was an instance where he could disappear and still be considered with you, which was not good, since it prevented people from starting Honest Hearts. We fixed the most common cases in our previous patch, but for people who had it occur before that, we've added the ability to get him back again. Also there was one more rare case where he could disappear, and that's been fixed.
There was also a case where he could die in non-hardcoe mode. That's been fixed too.
I looked on The Vault and discovered I'm not the only one with this bug. Is there anyway the glitch where it will suddenly become impossible to re-hire Arcade can be fixed.
Showing up with no followers causes him to reply that the Courier has "quite the menagerie" at their disposal, and talking to him with just Rex or ED-E but no other followers yields no conversation options relating to hiring him.
It seems kind of trivial, but it has happened to me on 3 out of 3 playthroughs (well 2.5 out of 2.5, but that was thanks to the Strip hat glitch that is now fixed), so it's getting a little tiresome.