A player over on the Diablo forums wanted some clarification on the seasonal stash tabs. Nevalistis gave some helpful clarification.
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If we don’t unlock the seasonal stash tab with seasonal conquests, does it unlock anyway after the season ends or are we SOL on that stash tab forever?
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. 😉
[source]http://us.battle.net/d3/en/forum/topic/20742996117#9[/source][/blue]
At least now you wont have to worry about the extra stash tabs if you miss a season in the near or long term future.