Yup. TIM goes on the die to transfer heat to the Integrated Heat Spreader (which is the lid you normally see). Intel has an issue with their thermal paste, it doesn't properly fill the void and doesn't contact the IHS well, so heat just builds up in the die. Check out this amazing chart from the Tom's Hardware review:
Heat just is not getting to the IHS properly on these chips, and heavy overclocking just makes the whole thing worse.
It's really been a slow-burning problem since Ivy Bridge, where Intel switched from soldering the lid to TIM + an adhesive. Solder has long been preferred for its superior heat transmission, but Intel says say smaller dies have problems with cracking over time due to thermal cycling. However, AMD has been happily soldering the lid on much smaller Ryzen dies, so apparently it's not all much of an issue in practice.
Well, you can get away with that on a processor that puts out 50 watts during normal operation. It's been an issue for a while on the unlocked/overclockable SKUs, particularly on the latest 7700Ks, but even an OC'd 7700K only puts out ~100W, so it was relatively manageable. Extreme overclockers could delid and replace the thermal paste with something better (often liquid metal like Conductonaut), which does help performance quite a bit.
But, with the higher TDP of Skylake-X, this has become a pressing issue just for normal operation. Things change when we're talking about a $1000 processor that needs to be delidded to sustain boost at stock settings. That's just not acceptable.
I almost would rather have a bare die at this point. Mounting pressures are no longer insane so it wouldn't be as terribe an ordeal to mount as as Athlon XPs were back in the day (god forbid your screwdriver slip on that bracket, with 50+ pounds of force you are guaranteed to gouge something).
TIM = Thermal Interface Material