For years, Samsung has sold their Galaxy flagship devices in two major variants: one variant with their in-house Exynos system-on-chip and another with the latest Qualcomm Snapdragon system-on-chip. The former is sold internationally, while the Snapdragon model is sold in select markets—usually in Hong Kong/China and the United States. Unfortunately, the model sold in the U.S. is usually impossible to bootloader-unlock, so the only way to get a Snapdragon Galaxy flagship with an unlockable bootloader is by importing one from China. This year, however, it appears that the Samsung Galaxy S9/S9+ with the Qualcomm Snapdragon 845 is being sold in Latin America, too, which is great news as we now have confirmation that AOSP-based custom ROMs such as LineageOS 15.1 can run on them.
When the latest Samsung Galaxy flagship pair was made official, we feared that only a handful of users would be able to grab the model with an unlockable bootloader. That’s a shame because there’s a lot to love about the hardware, though Samsung Experience might not be everyone’s cup of tea. On the other hand, the Exynos 9810 model is able to run AOSP-based ROMs such as this generic build of LineageOS 15.1 as the international Galaxy S9/S9+ models have unlockable bootloaders. This is, of course, thanks to Project Treble compatibility, which allows for devices to support flashing Generic System Images (GSIs) of Android.
Credits: XDA Member desudecchi
If you have either the SM-G9600 or SM-G9650, which I believe are the Snapdragon Samsung Galaxy S9 and Galaxy S9+ models with unlockable bootloaders, then you can install AOSP ROMs from our Treble-Enabled Device Development forum. There’s no official TWRP build you can use, but XDA Senior Member travis82 has put together an unofficial build that you can install on your device to make backups and flash the GSI. XDA Member desudecchi confirmed this on their own device, whose screenshots I embedded above showing off LineageOS 15.1 on the Galaxy S9/S9+.
Thanks XDA Senior Member travis82 for the tip!