The support for has many advantages, one of them is the ability to unlock the bootloader on the device enabling you to flash your favourite recovery, such as TWRP. However, some of the models of Galaxy S9 and Galaxy S9+ have their bootloader unlocked.
A Senior XDA Member travis82 has made available an unofficial TWRP version for the Galaxy S9 and Galaxy S9 Plus running the Snapdragon chip. So users with this configuration can now have their favourite recovery, such as TWRP, and to install various things compatible with your model.
You can download the unofficial releases from the links below for both Galaxy S9 and S9+.