You know from time to time the concept of "deciduous" keywords and mechanics comes up as a corollary to "evergreen" keywords and mechanics? How it refers to a mechanic that keeps coming back, but only every few years? Here are twelve keywords and mechanics I believe to be deciduous. In every case, each has been used on at least two planes.

Cycling
Kicker
Vehicles
Transform
-1/-1 Counters
Poison
Convoke
Flashback
Daunt
Protection
Split cards
Hybrid mana

Thoughts?

FWIW, here are what I consider evergreen.

I've struck out the ones that I know are actual mechanics that come back as set mechanics and not Deciduous.

I would consider -1/-1 counters a set mechanic as well, because their inclusion has other set effects, i.e., prevents the use of +1/+1 counters.

I also think split cards are a set mechanic because they're never just included. I would include transform if it weren't for Magic Origins having just the cycle of five and even then. Anything that changes the frame, basically.

Maybe I have a more restrictive spectrum for 'deciduous'.

Colored artifacts. Add that to the list.

I would argue that if a "set mechanic" keeps re-appearing, it's deciduous.

Azure, let me ask you this: Can something go from being a set mechanic directly to evergreen, or is it deciduous first? For example, scry. Before it was evergreen, it was what you call a "set mechanic" and what I call "evergreen".

The definition MaRo uses for deciduous is that it's not in every set, but every set could have access to it if needed. And when it is used, it doesn't have to be a big theme for the set; it could just be one cycle or even one card. By that definition, several of those wouldn't qualify. Cycling has been reused quite a few times, but always as one of the set's big mechanics. Some of the others haven't even been reused that often (convoke, for example, was only in three sets: its introduction in Ravnica, Future Sight, and a revisit in M15).

Also, mixing keywords, keyword actions, and "mechanics" is weird. Things like "draw" and "tap" are just part of the vocabulary of the game. It doesn't make sense to list those unless you're somehow going for completeness, but then why leave out "cast", "shuffle", etc? And if you're also including unnamed abilities ("daunt"), there are countless others you could include (see this article on the mechanical color pie, for example).

Whether is evergreen, deciduous, or set mechanic is something they decide when designing the sets. Scry was a set mechanic. Then they decided they like it as evergreen, so that's what it became.

Yes, it can. As noted above, a Deciduous mechanic is one that is just a tool that any set can use (generally in small doses) as needed but isn't really used at the density to qualify for a full set mechanic.

As an example, Transform iffy because of Origins and now Ixalan (where it only appears on ten cards), but ten may be enough to get it over the Deciduous hump in to "real mechanic" territory.

Transform is in a bit of an odd situation. It has lots of design space and could fit into any set, but the higher printing costs usually prevent that. So when they do use it, even for a small number of cards, it's splashy.

One thing to note though, since MaRo has been really hammering home that R&D will only be concentrating on three mechanics per block, and Ixalan has Explore, Enrage, and Raid as the main mechanics, that means that stuff like Crew and Transform that are also in the set are expectantly deciduous.

I'm just going to throw out a possibly pedantic distinction. Maro has hinted a number of times that the DFC design space is enormous, and that transform is only a portion of the available space: we've even seen this with meld, which was a double-faced mechanic totally separate from transform. So I would classify DFC's as deciduous, but transform as a set mechanic.

I think Maro has also confirmed that Vehicles are deciduous.