#9 - April 2, 2016, 6:21 a.m.
While stash space won't unlock for players automatically at the end of Season if the requirements haven't been met, failing to acquire a stash tab reward in a Season does not permanently prevent you from acquiring it in the future. Seasons going forward (for the foreseeable future) will have a stash tab reward; there is simply a cap to how many of them you can earn, that being 10 total tabs.
For example, one player could earn a stash tab each in Seasons 5, 6, 7, and 8 (that being the fastest way to to hit the cap). Another might earn them in Seasons 6, 7, 8, and 10.
And yes, I know what day it is. No joking around here. ;)