The temperature compensation algorithm only covers the -40C to +85C range. The device can actually survive up to 125ºC C with no issue. The problem will be that we will not compensate for any changes in timekeeping that will occur when the temperature exceeds 85ºC, which is again where design stopped the compensation algorithm. In theory, we could make a part that compensated up to +125C, but the design definition for the DS323X parts was for up to +85C.
So we do not recommend customers use the DS323X parts at temperatures above +85C, not because of any gross failures, but because the algorithm stops compensating for temperature variations above +85C, and our accuracy drops noticeably at those higher temperatures.
You can do tests to characterize the accuracy of your part above 85ºC.