Generally speaking you still depend on the device manufacturer to ship a compatible vendor partition in order to run a given version of LineageOS. This means that if your device doesn't have an official version of Android 8, then it will never be able to run LineageOS 15.
Generally speaking you still depend on the device manufacturer to ship a compatible vendor partition in order to run a given version of LineageOS. This means that if your device doesn't have an official version of Android 8, then it will never be able to run LineageOS 15.