Is ninjutsu or ninpo taijutsu considered to be a koryu?
Many ninja clans were seen as rebel groups. Also the Tanaka clan a family sect of Koga Yamabushi Ryu Ninjutsu which also.has roots to the Yagyu Shinkage Ryu Ju Jutsu.
Shidoshi Senzo Tanaka told Hanshi Frank Dux that family sects like the Tanaka clan will.never het the honor it should have. So is in the Bujinkan. The Bujinkan has 3 ninja schools, and the rest are Samurai Ju Jutsu schools.
Some Samurai school of the Bujinkan are Koryu. But the three ninja schools: Togakure Ryu, Gyokko Ryu, Gyokushin Ryu are NOT Koryu.
The Koga Yamabushi Ryu Ninjitsu is also NOT a Koryu school of the Ninja.
As one aspect of the martial arts, the equivalent, more or less, of military intelligence, this is certainly a legitimate area of study, and techniques are included within the curricula of several comprehensive classical systems. But ninjutsu simply no longer exists as an independent ryu-ha or art. What is commonly taught as ninjutsu, in Japan and elsewhere, is a fairly recent collection of unarmed and weapons arts, two of which are independent koryu. This does not mean that these arts are not technically valid or that they don’t have historical provenance. But, the modern arts taught as ninjutsu and ninpo or budo taijutsu (whichever version of the name you favor) cannot be considered koryu per se.