Bujinkan (?) seems to be a martial art that appears in different flavors with regularity. Is there a reason why there is not a tag for it? Is Bujinkan already a subset of an existing tag?
There is a bit of crossover with
ninjutsu. I'm not certain how much we break away specific styles, especially since I suspect many people don't even differentiate. I would have no problem with it as a tag. We probably have about a dozen questions relating to it, and we have some subject-matter experts on the topic.
As regards the question in the comments about how exactly it fits in as a style, I'll be honest that I think it's somewhere in between a branch (like how we have a
Shotokan tag to distinguish from
karate) and marketing (my understanding is that you can't represent yourself as a Bujinkan school without being part of the organization, unlike, say, "Gracie Jiu-Jutsu" where the feuds between family branches have left the trademarks in dispute, or the broader "Brazilian Jiu-Jutsu" where there's even less restriction).
Lastly, while the Bujinkan system is typically thought of as a "Ninjutsu" school, only three of nine traditions of the school are claimed to be from ninja training, with the other six being from samurai training, so I think there's a good argument for it being supported as its own system, and tag, much like the various hybrid styles like Systema.