There are a handful of supertypes in Magic the Gathering, from highly recognizable ones such as "Legendary" to more obscure ones like "Host", but there's only one supertype that's appropriate to talk about during the holiday season: the snow supertype. The snow mechanic was introduced all the way back in 1995, less than two years after the game's original release, but due to a number of different factors the supertype never really took off. Let's step back in time to take a look at where it came from, why it failed, how Wizards of the Coast tried to revive it, and where snow stands today.
The mechanic originally came into existence for the Ice Age set, which was released in June 1995. This set was the first standalone expansion for Magic, and introduced a number of different mechanics, such as cantrips and cumulative upkeep. In this expansion, lands were the only cards that could be designated as snow cards, which at the time were referred to as "snow-covered lands" instead of just "snow lands" due to the naming convention at the time. While other cards would reference snow lands to determine their effects, these cards were not necessarily snow cards in themselves. At least, not at the time.
In this expansion, snow was introduced not for gameplay or mechanical reasons, but mainly for thematic reasons. The team at Wizards of the Coast wanted something that would fit the theme of Ice Age, and so snow-covered lands came into a half-baked existence. This is why the mechanic was so underdeveloped at this time, and why it was used on so few cards; out of the 363 non-land cards used in Ice Age, only 28 of them make use of the snow mechanic at all.
Furthermore, the developers made sure that the number of cards that punished the player for having snow lands was the same as the number of cards that rewarded the player for them; this is because they wanted to discourage decks that exclusively used snow-covered lands, as without punishment these lands would be strictly better than normal basic lands. This wasn't helped by the fact that certain cards in Ice Age scaled with the number of snow lands that were currently in play, which made balancing more difficult.
Ultimately, the snow mechanic was considered to be a failure at this time, and the design team had no desire at all to revisit it. If it weren't for the decision to create an Ice Age "block" thanks to adding the Coldsnap set to Ice Age and it's follow-up, Alliances post-mortem, snow would have immediately disappeared for a long time -- maybe forever.
When it was decided that the Alliances set (released in June 1996) was going to be part of the Ice Age block, the design team realized they needed to make some thematic connections with the Ice Age set itself. As a result, they decided to add... zero snow cards. All of the snow cards that appear in Alliances were added during the development phase, after the design team had done their initial work on the set. There are 144 cards in Alliances that don't appear in Ice Age, and a whopping four of them have any reference to snow lands.
After this point, the mechanic was basically forgotten about by the team, though it would have an influence on another set in a few years...
While not snow/"snow matters" cards, the idea of a type that had no special rules on its own, but would be referred to by other cards, returned in the Champions of Kamigawa set upon its release in October 2004. This was known as the subtype arcane, which led to the creation of the splice onto arcane mechanic. If a player cast an Arcane spell, they could tap some more mana to "splice" another spell onto that arcane spell, so long as the second card had splice onto arcane available. This allowed the first spell to take the effects of both spells, while the spliced card remained in the player's hand to be used again later.
Like the snow mechanic, the splice mechanic ended up disappearing from Magic for quite a long time. The problem was that the arcane limitation meant that splicing was too "parasitic", as it only worked with cards from the same set, which is not what Wizards of the Coast wanted. After a 15-year absence, it was reintroduced in Modern Horizons in June 2019, as two cards contained a mechanic called "splice onto instant or sorcery", removing the need for the spell to be arcane. Arcane cards returned throughout the Kamigawa block, and have been seen in various reprints, but the subtype has not returned since. This was not the case for snow, however.
Coldsnap, released in July 2006, was designed to be the "lost" third set of the Ice Age block. In order to make the set feel like it fit, a number of old themes and mechanics were reintroduced. The design team was reluctant to bring back snow-covered lands, but designer Mark Rosewater had a bold vision: instead of being bad, what if snow was... good?
Coldsnap was the first time the snow mechanic was taken seriously by the design team, and this is best exemplified by the creation of the snow mana symbol, as seen in the header image of this article. This marked snow as a new type of mana, but not necessarily a new color; snow lands would produce mana of the same color as they normally did, that also happened to be snow mana at the same time. This further led the team to create cards with snow activation costs, requiring a snow mana specifically in order to work. On top of that, the term "snow-covered" was replaced with just "snow", as this allowed for more space on cards.
A number of other critical decisions were made, in order to best balance the snow mechanic. For the most part, cards were not dependent on snow lands or snow mana to be good, but having a snow mana often made them better by allowing their abilities to be activated. Additionally, there were very few cards introduced that scaled directly with the number of snow lands in play; some scaled with snow permanents, but this could include the newly-created snow creatures, artifacts and enchantments, none of which required snow mana to summon. As a result of these changes, Limited players didn't need a large number of snow lands in their decks to make snow cards viable, which was one of the weaknesses of Ice Age.
The dedicated design work for the snow mechanic made it much more prevalent in Coldsnap than in previous expansions. Out of the 155 cards that were introduced in this set, 57 of them (more than a third) were snow spells or made reference to snow spells. There was even a pre-constructed deck built around the snow mechanic called "Snowscape." So why has snow made so few appearances since then?
According to Rosewater, the problem once again lay with the snow basic lands. There was simply too much confusion on the rules of snow basic lands, specifically with how many of them could be used in a deck. Ultimately, snow basic lands ended up being unofficially and seemingly banned from Standard play, which more or less sealed the fate of the snow mechanic. As a result, Rosewater considers these basic lands to be a "mistake", and he doubts that the snow mechanic will return in any Standard-legal set in the future. However, not all Magic sets are created equal, and not all of them are created for Standard play.
Released in June 2019, Modern Horizons is a set that is specifically designed for Limited and Modern play; this means that a number of mechanics that are outlawed in Standard were fair game for this new expansion, including snow. Of the 254 new cards in this expansion, 23 of them utilize the snow mechanic, including the five snow basic lands. The rules of snow did not meaningfully change in this expansion, though this is the first time that snow mana was used as part of a card's casting cost (even if it only applied to Icehide Golem and Arcum's Astrolabe).
As of now, that's where the snow mechanic exists today: (unofficially) banned in Standard, but legal in Limited and any other format that allows Modern Horizons and cards from the Ice Age block. It's unclear when or if it will make another appearance, but it was certainly an interesting experiment while it lasted. It's possible that its influence will be felt elsewhere. Until then, we'll always have drafts and those ever-so-popular non-rotating formats.