As far as I know you can still turn it in.
But yes, some people are selling it because now there's a cake decoration that is reliant on having the corresponding memento.
Sell it. You stand to make millions if I understand correctly.
In-game username: Un1korn | Happy member of the PCNA UESP guild (Resident Daggerfall Covenant enjoyer) | Main & basically only character: Crucian Vulpin, Imperial Dragonknight of the Daggerfall Covenant, and Undaunted Bulwark (I tank) | Mountain bike enjoyer and vulpine appreciator | If you know me from PCEU: No | To ZOS: THANK YOU FOR LETTING ME BRING MY HORSE INTO BATTLE!
I'd use the voucher, if it were me, but it probably depends how into furnishing you are.
I used the one I got a couple of months ago.
When I first got it, it didn't work, but then I read somewhere (to my relief!) that it only worked while the event was on.
So, I waited and sure enough, it did work:
As you can see, you get the original, single tiered, 2016 cake.
You could wait to sell it later, but whether it will be worth less, or more, by next year, who knows?
Also, people have them listed for extremely high amounts, especially on PC NA.
However, whether they will sell for that, again who knows?
Logically, they should be worth more than they were when they were only for the momento (and most people didn't know they could even be used for that), but that doesn't mean they're going to be worth 10 million+, to most people.
Or that the people, who really want them, can afford to pay that.
Most avid furnishers are not rich, contrary to popular belief, as they constantly spend all their gold on furnishings and/or on mats.
So, however much they might want the voucher, most can't afford to pay a lot for it.
On PC EU one is, currently, up for 1 million, for example.