Hey ! Welcome in the community forum 
Indeed the warning message is a little scary 
But I think it’s just here to avoid any issue. I doubt it could really make your device unusable. And this text is here for every devices, even those that aren’t on Android Pie or 10.
It says not to install /e/ Oreo on a Android 10 phone, but it doesn’t say not to downgrade the stock ROM by another stock ROM.
The Galaxy S9 was released with Oreo, so you could download the last Oreo version of the official firmware, and then install /e/.
Before you start anything, also download the last Android 10 firmware for your device in order to be able to restore it quickly in case it doesn’t work.
If you choose to take the risk, here is a HowTo you can use :