While there isn't a whole lot of extensive criteria for determining why a jutsu has been deemed forbidden by the ninja world of "Naruto," there are three major qualifications that most kinjutsu fulfill at least one of, according to the "Naruto" wiki. The first is that a jutsu can be deemed forbidden if it has a high risk of harming the user. The Demon Dead Consuming Seal, for instance, requires that the user sacrifice their own life, making it forbidden.
Another qualification is that the jutsu violates the laws of nature. Pretty much anything that resurrects the dead, like the Outer Path — Samsara of Heavenly Life Technique, falls into this category. Finally, if a jutsu is likely to result in collateral damage, then it will be deemed forbidden. This final criteria is the most likely reason that Naruto's iconic Multiple Shadow Clone Jutsu is considered forbidden, as the latter two stipulations rarely apply to it.
Oddly, just because a jutsu is forbidden doesn't mean that people can't use it. While it may not be openly taught or practiced, there is no real societal ramification for using a kinjutsu specifically. Otherwise, Naruto would have been locked up after learning the Shadow Clone Jutsu in Episode 1. As the wiki points out, however, creating or studying these jutsu in an effort to further them is considered punishable, causing certain rogue shinobi like Orochimaru to leave their respective village.
