First Nicolas has said the drop rate is unchanged and there is no reason to believe that is not true.
Second Myself and my other players who have run thousands of simulations have seen a sharp drop off in the rate.
1st Possible Reason
The drop rate could be a fixed number of I Cores per Sector per Day. So if kabam is going to give away 1000 I cores a day in a sector. The first couple days when only the plat players who accelerated their current builds and had defense strong enough to play lots of levels had higher drop rates. As more people in a sector began getting the simulator and playing simulations the chances of winning one of the 1000 I cores decreased because simply more people were playing. Kabam may not have changed the number of I Cores being dropped the rate for each player decreased.
2nd Possible Reason
The I core could be assigned to a random level in the simulator each simulation. When you simulator is at level 1 or 2 there are a lot fewer level the I core could be behind and there for the drop rate seems higher. As you unlock more level there are more simulations and a lower chance of getting an Icore. Again kabam would not have changed anything but the rate would decrease a lot. Test this you could deconstruct your simulator and start over to see if the drop rate goes up again. I would not recommend doing this unless your willing to test out the idea because i have no way of know if this theory is correct or not.
3rd Possible Reason.
Kabam is going to love me for this idea if people test it. I am only putting it out there because of a personal experience. Yesterday i spend the tournament plat i had been saving. All of it on I cores. Shortly after i did that my drop rate sky rocketed to 1 in 30 to 1 in 60 and after a couple of hours it returned to 1 in like 500. I don't know how kabam would program this but the drop rate could be like to how recently and what amount of plat you spent in the shop. Again i don't recommend you doing this because i have no proof what so ever that this would work. Again if this is the case kabam wouldn't have had to change anything the rate would just naturally decrease the longer you didn't spend plat. I have a feeling in the beginning there was a lot of plat spend to accelerate the builds of the Defense Simulator.
If you have any other idea that could cause the rate to naturally decrease without Kabam changing anything please post them. If you test any of the ideas and are successful or unsuccessful please post your test and your results.
Nicolas if you could confirm or deny the accuracy of any of these ideas i'm sure many people would be very happy. Or even better told us what the drop rate was based on. Like the A1 its nice to know that you need to hit around 400 higher then 1/3 your level and less than your level. Any hints would be greatly appreciated.