Lobotomized kitten or no, the fact of the matter is, you had to be standing perfectly still to be hit by a Backstab. In or out of combat, you'll still outpace the stealthed person, and to actually hit someone with that from behind to do real damage (instead of a sneeze), it wasn't really feasible against anyone that knew what they were doing (and if you somehow didn't see a Thief coming before they stealth, I would have to commit blasphemy by telling you to turn the porn off while you're PvPing at least). It's weird to say, but even just running forward keeps your back the least exposed, neutering Backstab's effectiveness.
It was a little too powerful under conditions in WvW that shouldn't/will never be balanced. But the only time a person of any skill should be dying to a Backstab crit is if they were already outmatched by something else like a Warrior stun (but the same could be said about Hundred Blades being used effectively by a Mesmer pistol stunning you first).
They didn't actually change Heartseeker again, did they?