First of all I have to say English is not my native language so I can say things that can be misunderstood, but since now I don't think this is the case...
I never said or meant that US and International Samsung phone are the same or could be treated the same way nor did I said that ALL US phones could be unlocked, but as you said yourself : "
A few variants were unlockable for a bit like tmo or sprint but for the most part for years and years us models havent been unlockable without exploits and even then a majority of said exploits were just for root (bootloader was still locked and not able to use TWRP.)"... ^^
Then it's hard to know when you participate into a discussion as moderator or not as you always use the same pseudo with the same status "elliwigy : Forum moderator /Recognized Dev / Dev Relations"... ^^
But when we go in the Snapdragon dev section for the Galaxy Note 9 your name appears seven times in seven threads... Excluding the open thread where you explained how to root the Note 9 with an exploit.
Samsung Galaxy Note 9 (Snapdragon) ROMs, Kernels, Recoveries, & Other Development
forum.xda-developers.com
About unlocking the bootloader of the US Note 9 in 2018 I was indeed confusing with the SD 9600 which is a Snapdragon but not for the US model... ^^
But if you go to this following thread OP, did say the US Note 9 could be unlocked with possibility to install TWRP and it was by May 6, 2020 even if the title mentioned N9600 !!!
This ROM is based on Samsung Note 9 TGY firmware and modded to allow full US carrier functions (volte, wifi-calling, etc.), but can be installed on any N9600 that meets the requirements below. Flash at your own risk! I am not responsible for any...
forum.xda-developers.com
Requirements:
- Must be on Android 10 firmware. I recommend that you be on 5FTK1 bootloader
- Must have klabit's official TWRP 3.4.0-0 installed
Instructions:
- Make a full TWRP backup!!
- Download ROM to phone from link below
- Download ODM to phone from link below ONLY if you're on a USA carrier
- Reboot to TWRP, select wipe and swipe to factory reset
- Install ODM if you're on a USA carrier
- Install ROM
- Reboot
- After setup, open rom control, open each tab in rom control and reboot SystemUI
*For SafetyNet to pass, you need to enable Magisk hide in Magisk manager settings
Downloads:
Magisk seems to work fine...
Then there is another and newer thread from afaneh92 (Nov 30, 2021) describing how to install TWRP on US Note 9 (using a paid service to unlock the bootloader is another subject and it doesn't matter)
Team Win Recovery Project 3.x, or twrp3 for short, is a custom recovery built with ease of use and customization in mind. Its a fully touch driven user interface no more volume rocker or power buttons to mash. The GUI is also fully XML driven and...
forum.xda-developers.com
Finally, you admitted you owned a "My S21U did have an unlocked BL"... So what should we think about your own words ???
In regard with my own International A528B phone I did the unlocked process and tried many times to install a custom AP file with Magisk but it didn't work as my phone enter into a bootloop process after flashing it via PC Odin. At last resort I relocked my phone and I was surprised to receive an OTA update which should be impossible if my phone was really unlocked !!! ^^
How can you explain this ??? lol