The current /e/ should install on a s9, at least I did it last month with an up to date second-hand (It was at least android 9, might be 10) s9. Only a bit fiddly getting the TWRP to install because of the security measures of Samsung. This thread also list people installing it on the pie version Help installing /e/ on Galaxy S9.
Though the wiki for the s9 now says you shouldn’t downgrade, it didn’t say that last time I looked. It is probably added by mistake, it was for the s8 according to this post Samsung Galaxy S8 - warning.