The Nexus 5X Will Stay on Android Oreo
The Nexus 5X Android 8.1 update will be one of if not the last major updates the device officially gets from Google, but that doesn't mean support will end.
Google originally said it planned to stop security updates for the Nexus 6P and Nexus 5X in September, 2018. The company changed those plans in 2017.
Google now says it plans to roll out security patches to the Nexus 5X through November 2018 giving you several more updates to look forward to. After that, support for the Nexus 5X will stop and you'll be on your own.
Unfortunately, the Nexus 5X won't get Android 9.0/Android P. Google says there are no guaranteed Android version updates for the Nexus 6P after September, 2017 and the company's Android P beta confirms those plans.
The Android P Developer Preview is currently live for all Pixel models except for the Pixel C tablet.