Are the unique cards from the planeswalker decks (e.g. Flame Lash, Nissa, Nature's Artisan) legal in constructed formats other than Standard? Logically, the answer seems like it should be yes, but every resource I've found that references them, from the WPN alert to Maro's more in-depth introduction to the MTR itself only mentions that these cards are “legal for play in the Standard format”. Are they legal in other formats too, and if so, is there a reason for calling out Standard every time?

Gatherer shows it as legal in all constructed formats including modern and legacy. I think the wpn alert was just highlighting that the cards were legal in standard even though they aren't in any booster packs. The same was true for shivan dragon and a few other cards in origins. The cards are to be considered part of the set they have a symbol for and are legal in formats that include that set.

Originally posted by Preston May:

The same was true for shivan dragon and a few other cards in origins.

FYI, these cards from Welcome Decks are still legal for the current Standard (this year's Welcome Decks are the same as the last year, AFAIK).

I think MTR 6.6 (Legacy) and 6.5 (Vintage) cover this just fine.

Originally posted by MTR:

Cards from expansions and special sets (like From the Vault, Magic: The Gathering—Commander, Duel Decks, Conspiracy, etc.) are legal in the Legacy format on the date of release of the expansion or special set

I'd imagine the reason Standard is specifically is called out is more Standard events are run at store level than Legacy/Vintage.

Originally posted by MTR 6.3:

Planeswalker Decks contain ten (10) cards that, while not appearing in expansion Booster Packs, are considered part of the expansion released concurrent with those Planeswalker Decks

I'd imagine that answers if they are legal in Modern as well.

Originally posted by Preston May:

The cards are to be considered part of the set they have a symbol for and are legal in formats that include that set.

Shivan Dragon has the set symbol M16 though, which is neat but a bit confusing.

if you are unsure about a card's standard legality, you can always check magiccards.info, you can also check foreign names of cards.

It has not been updated yet to the new block :(

Originally posted by Olivier Wattel:

if you are unsure about a card's standard legality, you can always check magiccards.info, you can also check foreign names of cards.

as i am a fan of magiccards.info, they are a bit slow when it comes to release dates :)

http://gatherer.wizards.com/Pages/Default.aspx

This is for release stuff a bit better ^^

Magiccards.info is indeed a useful tool, however, we've had issues with it in the past due to the speed of its data base updates (and sometimes even errors). If you need a quick reference, I suggest using an official oracle DB: http://gatherer.wizards.com/