Yes, Intel* XMP is considering overclocking, altering clock frequency or voltage may damage or reduce the useful life of the processor and other system components and may reduce system stability and performance. Product warranties may not apply if the processor is operated beyond its specifications.
You can check this information in our warranty terms and conditions "any Product which has been modified or operated outside of Intel’s publicly available specifications, including where clock frequencies or voltages have been altered, or where the original identification markings have been removed, altered or obliterated. Intel assumes no responsibility that the Product, including if used with altered clock frequencies or voltages, will be fit for any particular purpose and will not cause any damage or injury. "
The Intel® Core™ i5-9600K Processor works with DDR4-2666MHz and that's the speed we recommend you to use.
You may find useful the Tunning Protection Plan, The Performance Tuning Protection Plan (PTPP) is an additional plan to cover processor failures caused by operating the eligible processor outside of Intel’s published specifications. You can check for more details in our FAQ's
If at any time you have issues with the processor since you were not aware that XMP* was considering overclocking, the best would be to contact the Intel* Live Support. https://www.intel.com/content/www/us/en/support/contact-support.html#@2