Hi all,
I had a terrible doubt!
Unspent tavern tickets gained from current reward track can be kept between “years” and expansions, or they will disappear?
Thanks in advance
If you mean the things that can be exchanged for arenas or heroic duels, they can be kept 
I even heard suggested to start arena run: you will get the ticket back + reward for starting the run, but I have not tried that myself yet.
True in a very limited way.
On March 30, when the new expansion releases; the Core Set is introduced; and the next Wild rotation happens; all unfinished Arena runs will probably be force-expired.
I say “probably” because this force-expiration of unfinished Arena runs is usually done when the available sets for Arena drafts chance. But I have not seen an explicit announcement of this at this time.
Whenever an Arena run is force-retired instead of ending naturally, you get the prices for the wins so far (which might be less than the potential of the run), and a ticket for a free Arena run as compensation for the forced early retirement. This goes for all unfinished Arena runs, regardless of whether they were bought with a ticket, with in game gold, or with real money.
If you want to min-max your in game rewards, then it certainly helps to have an unfinished Arena run whenever force-retirement happens. And the true min-maxer wil make sure to have as many wins as possible without accidentally ending the run (which in practice means: play until you have 2 losses, then stop and wait until after force-retirement).
But this is unrelated to the Tavern ticket.
It will as the new set will be added and that’s caused the force retire every time.