Just as it did after the unveiling of the Galaxy S8/S8+, Samsung has published the kernel sources for the Galaxy S9 and Galaxy S9+. But this time around, the company has gone ahead and released the code for the Exynos and Snapdragon models at the same time.
Of course, these won't be of much use to the everyday consumer. To a developer, though, this means that custom development can commence. Unfortunately, Samsung phones --- especially those with Snapdragon chipsets --- usually don't see too much activity in that arena, though the Exynos S8 does have TWRP and unofficial Lineage OS.
The kernel source codes can be downloaded from Samsung's Open Source Release Center (OSRC) below.