Not really forbidden, just guarded by honor and competitiveness.
It's just like if you ask someone who always wins in competitions and such, and you ask what his secret is, chances are he won't tell you. Because it's his technique that is earning him wins and he doesn't want anyone else to know it.
Generally it's the same thing. Hundreds of years ago, sword work and hand-to-hand combat were basically the only attack and defense of militaries and civilians. Countries would come up with their own killer techniques to kill their enemies. Of course they were not gonna tell them.
This legacy is still carried on, many techniques are lost to humanity because the people who knew them all died out without teaching anybody their secrets.
That's why if you travel to Asian countries to learn a martial art, you may get the chance to lear some very secret techniques. These are only taught to the best of the best students that not only show skill, but show that they are trustworthy.
An example of a small style of these techniques are know to mainly Shotokan Karate Grandmasters, but they do teach some students if their lucky.
It is a small list of techniques that can stop the heart with a single blow. Before you go on about how it's impossible, it's not. A few years ago in Canada, an 18 year old Ice Hockey player got hit to a vulnerable area and died from a single shot that stopped his heart.
I cannot tell you the exact technique or area to attack for these techniques, as I do not know them myself (I wouldn't tell you anyway :P) but they are open hand strikes somewhere around the heart.
As for 'Forbidden' techniques, they do exist I guess. Examples are the arts of Muay Boran and Pankration.
Muay Boran was the ancestor of Muay Thai. It was a series of deadly killing techniques and vicious attacks that was outlawed sometime at the start of the 1900's. Then all Muay Boran masters were forbidden from teaching Muay Boran techniques, but then the cleaned up sport of Muay Thai came out. It had certain rules and protective gear and was minus of the killer techniques.
Muay Thai was legal, but Muay Boran was not legal again until recent years, where people have tried to re-create the art, as most masters had died.
So with Muay Boran, some techniques were legal, then forbidden and now lost.
Now for Pankration.
Pankration was a Hybrid combat sport that was one of the most popular sports in the Ancient Olympic Games. It was brutal. When the Ancient Olympic games came to an end. Pankration was outlawed. It was only taught to the Greek Military.
Eventually it died out, with the techniques lost to humanity. However, in the 1990's historians found carvings of Pankration in stone, and Martial Artists re-created the art. However, about half of the techniques cannot be recovered. Once again, was once legal, forbidden and now lost.
Hope this helps.