The max thermal limit for the 1080 Ti is 91c, so you're well under that.
https://www.nvidia.com/en-us/geforce/products/10series/geforce-gtx-1080-ti/
That said, the thermal throttling temp of a 1080 Ti is 84c, and you are very close to that. The reference Nvidia GPUs are not as well cooled as the ones with aftermarket coolers. Otherwise it might be peaking at more like 70-75c.
If you're concerned about it possibly peaking higher and throttling, maybe install another intake fan on your case if you can, like on the side panel blowing on the GPU.
As for possible damage, no worries. Both CPUs and GPUs have built-in thermal limits, whereby they throttle down in frequency when that point is reached. It's there to prevent them from overheating and getting damaged. It CAN however drastically affect performance should they reach that temp though.
The max temp of the i7 8700k is 100c. I cannot find any info on any specified throttling temp, or if there's one that's lower than that. Regardless, I assume since your apparent reference GPU is within it's throttle ceiling, that so too would be the CPU.
https://ark.intel.com/content/www/u...8700k-processor-12m-cache-up-to-4-70-ghz.html