The reason we don’t advertise it is because it is just brittle in practice: It needs the right combination of SSD and SSD enclosure as well as RPi 4 firmware. The USB stack (and USB mass storage) is more complex than SD card enumeration, which in the end just leads to more cases where things can go wrong. And they do quite often: Just check the GitHub Home Assistant OS issue tracker, recently most issues stem from USB SSD boot… All this just make USB SSD boot not something we can recommend generally.
We certainly won’t break it on purpose, and address issues where possible. But we cannot make all combinations work… So if your combination works, that is fine! If it doesn’t then you should consider booting off SD card and use datactl command to move just the data partition to the USB SSD drive.