Hello @pitorel, from your research you’ve come to the right conclusion: A downgrade from StockROM Android 10 #G965FXXU8DTC5 to StockROM Android 9 is no longer possible due to Samsung’s security guidelines for updates. In fact, you can no longer downgrade to StockROM Android 10 February or January 2020.
This red framed note hasn’t been displayed for at least half a year. Samsung simply dropped it under the table. But the consequences have remained.
If you don’t believe it, try a downgrade. Nothing will happen to your S9+. The initiated flash process is simply cancelled and you will receive an error message in red on the S9+ display, which goes something like this:
Odin Mode
Download Speed: Fast
Poduct Name: SM-G965F
Current Binary; Samsung Official
System Status: Official
FAP Lock: Off
Secure Download : Enabled
Warranty Void ....
RP Swrev:: ...
SW Rev. "Check Fail (bootloader) Device ...
When /e/ OS 8.1.0 Oero /e/ for star2lte dev (oreo) flashes on the Android 10 bootloader, this will result in the known boot loop under /e/ OS.
In this context, the warning notice on the /e/ dokumentaion says
Warning : Downgrading smartphones already running on Android Q […] to Oreo or Nougat can lead to instability or in worst case block some devices