The Nexus 5X and the Nexus 6P were announced way back in 2015. Both of these phones had their own series of issues, but more importantly, they marked the end of an era. The LG-built Nexus 5X and the Huawei-built Nexus 6P were the last Nexus-branded devices to ever be released to the market: they were succeeded by the original Google Pixel and Google Pixel XL, which served an entirely different market segment and targeted average consumers rather than just power users. Despite issues arising on both the 5X and the 6P, they both chugged along, receiving Android Nougat and Android Oreo officially as well as timely security updates. However, support for these phones is possibly ending now.
An over-the-air (OTA) update, which is based on Android 8.1 Oreo, is now rolling out to both the Nexus 5X and the Nexus 6P (as well as to the Pixel/Pixel XL, the Pixel 2/Pixel 2 XL, and Pixel 3/Pixel 3 XL), bumping up both phones to the latest November security patches. But if Google's update policies are to be followed here, then this will be the last update to be released to both Nexus phones, meaning that the Nexus 5X and Nexus 6P have now officially reached end-of-life (EOL) status. Furthermore, online and phone support for both devices is also being discontinued, as per this support page.
Going forward, this means that owners of these devices will not be getting any more updates from Google, not even security ones unless a critical vulnerability requiring fixing is discovered going forward. This was eventually coming, as neither phone received an update to Android Pie--Google's update policies for Nexus phones were two years of platform updates and three years of security updates, and it's now been over 3 years since the release of both devices. You will still have the option of using regularly updated custom ROMs like LineageOS if you want to keep your device updated from now on, or simply switch to another device.
If you're still using your Nexus 5X or Nexus 6P as your daily driver, then be on the lookout for your last update, which should be rolling out in waves now.