If I recall, the US S21 Ultra can b e bootloader unlocked via a service on Telegram but only on a specific Android version. After that version, you are out of luck.
I had an HTC 10 at the time, around its release date as that was the only phone with a Snapdragon 820 available at T-Mobile in my location. It was an ok phone but got better with a custom ROM+kernel. Shame about no dual front facing speakers on the HTC 10 because the single front facing speaker actually sounded great.
It's not like YouTube kids was able to serve its purpose anyway.