The only evidence I can really find that points to its possible existence is Wargaming's description of it which is something along the lines of "The E 100 was used as a base for a tank destroyer, a self propelled gun, and an anti-aircraft vehicle" but beyond that I can't really find anything. The closest thing I can find is the Sturmtiger and a bunch of railway guns and heavy anti-tank field guns, but nothing that resembles the Jagdpanzer E 100. I'm under the impression that Wargaming just theorized what it would look like based on the looks of the Ferdinand and JPanther II along with maybe the Jagdtiger, and then gave it the armor and mobility of an E 100. Further evidence that proves this theory is that the vehicle doesn't exist in War Thunder either. They have other fairly similar vehicles like the Maus, E 100, Ferdinand, and Jagdtiger (which did exist IRL) but no Jagdpanzer E 100.
I also tried Googling the name of the Jagdpanzer E 100's gun, which brought me to some field guns and artillery, but nothing like the Jagdpanzer E 100. None of the guns I saw even looked like the one on the tank. I also tried Googling the name of its ammo which brought up nothing to help me.
Am I missing something or am I right about Wargaming just coming up with the idea for it because they needed something for the Tier 10 German tank destroyer when they were creating that line a long time ago?