I think that for an OS like Lineage, which is supposed to be just Android-without-the-Google-bits, upgrading the major Kernel version will be next to impossible. The proprietary vendor blobs compiled for 4.4. will not load with 4.14 or later. That's just not how Linux works.
Yes, there may be some other ms8998 devices out there using later Android kernels. So it may boot somehow. But what's the good of that if your LTE-modem, camera, and touchscreen no longer work?