He was already kind of out of date by the early 2000's which I would say was his heyday: 1999 - 2003.
But really rap itself was changing around that time. Gangsta rap was already reaching its excess and indie rock was taking over. Rap was changing to dirty south and then trap, which is where it is now.
Although now every song has rap or a trap beat in it even as just a small part. It's almost comical. So in retrospect, anything coming from Eminem is still good because it doesn't sound like all the fake rap now.