Thank you for telling me! Time to buy those tabs :D.

they are shared in every league ever they will do, even in standard

All purchases are for your account.  If you buy a tab, every league has that tab.

Yes, you get a full set of all tabs you own on every single league.

Tabs are account wide, not league specific.

When a league starts, you will have all the tabs you paid for  + the free ones empty. when the league ends, the league tabs get moved to standard and become remove only. (Items can be taken from those tabs but nothing can be added to them, they are only deleted after every item is removed and a log out.)

I'd like to buy a few premium stashes, but I don't want to buy them for every league.

If I buy, and use a premium stash on Normal, will it be there on Legacy?