A BIOS update for Skylake mainboards makes it impossible to overclock Skylake CPUs. The update contains code from Intel that makes overclocking of Skylake CPUs only possible on K-series processors.
Many mainboard manufacturers decided to not limit overclocking to the K-series when the Skylake CPUs were introduced. Instead they supported overclocking on all Skylake models. This makes experimenting with overclocking much cheaper, the K-series CPUs are usually priced in the higher regions.
Intel isn’t happy with the manufacturers, the non-K-series CPUs haven’t been designed for overclocking and therefore can get damaged more easily. A new BIOS update that Intel provides to mainboard manufacturers restricts overclocking to K-series CPUs again. The update has to be performed by the user, which means that anyone who doesn’t apply the update can continue to overclock their Skylake CPU.
According to Intel it’s up to the mainboard manufacturers to add the restricting code to their next BIOS update, according to the chip giant it’s not mandatory. It’s likely that Intel will use a hardware based overclocking restriction in its next generation CPUs.