Is it true that hiden jutsu can be taught? I was browsing through naruto wiki, and came across this page
It is stated that ,

They are not to be confused with kekkei genkai, as technically, hiden techniques can be taught to anyone.

By &quot;they&quot;, it means Hiden Jutsu. Is it true that they can be taught to others? Is there any evidence of this happening?
I remember in the Boruto movie that the Shadow Jutsu is used through that advanced gadget. But is it possible for a person who does not have any clan relationship to learn a Hiden Jutsu?
I'd go with Yes.

Hiden Jutsu are hidden clan techniques that are passed only within a clan. The nature and need for secrecy arises only because they can be "stolen". Ergo someone outside the clan can learn these techniques. I would also guess that someone outside the clan can also be adopted and taught those techniques. If they meet the criteria they can also figure out by themselves. Lets take some examples.


Hyuuga Clan: Byakugan may be a kekkei Genkai, i.e. Someone only from Hyuga Clan can manifest these eyes. But Neji for example, just because of his natural talent figured out Hiden techniques such as 32/64 Palms, Rotation etc. Later his uncle taught them to him popularly. Now Ao may possess the Byakugan but unless taught he can't use any Gentle Fist techniques.
Uchiha Clan: Kakashi, Danzo etc. have shown to be able to use the clan's Hiden techniques once acquiring the Sharingan. These include Genjutsus, specialized techniques such as Izanami, Kamui etc.
Ina-Shika-Cho: We see other users using the "Reading of mind" on the ninja Jiraiya sent. But when Pain uses it on Shizune, Ino remarked on it as "Dad's Technique". I would think that such techniques are specialized by the Yamanaka clan which may or may not be passed outside. Nara Clan's shadow technique was not used outside the clan. Akimichi Clan probably requires the genetic body build to use the Body Expansion techniques.
Team 8 - Both Aburame and Inuzuka clan raise the children in such a way that they can use their Hiden techniques. The kids in Aburame clan bond with insects, while Inuzuka clan raise a puppy and grow together with them. 


Thus Hiden techniques are secret since each individual are brought up/born in a manner that they can only learn their clan techniques. 
If Iron Fist Style can be taught, why not the Gentle Fist with someone with precise Chakra Control? Because they don't have the Byakugan to precisely dodge and target the areas while fighting.