I am honestly not a fan of seasons but the extra stash space award is really useful (affects both season and non-season).
I recall blizzard changing stuff about season rewards in the past few patches but i do not remember what.
My diablo 3 is slowly being downloaded so i can not open up the game and read from there.
So what is currently needed for the extra stash award in Season 9?
Conqueror in the season journey.