The Galaxy S8 and S8+ have not been out for long but already have TWRP. Unfortunately this is only available for the Exynos variants of the devices. With TWRP you will be able to root the phone as well as flash Roms and other mods. The Galaxy S8 and S8+ do have dm-verity so you will need to flash a custom kernel that disables dm-verity before making changes to the device or your device may not be able to boot.
If you are not yet rooted you will need to flash TWRP using Odin. If you are already rooted you can use the dd command in terminal emulator or install the TWRP app from the Play Store.
As far as rom support is concerned Cyanogen has historically refused to support Exynos devices since source code is harder to get on those devices. Since most stock roms were built on Cyanogen it meant that there weren't many options for Exynos devices. Who knows if Lineage will make the commitment to support these devices.
via TWRP
