Great question. I suggest that many of the so-called "too dangerous" techniques are very low-yield in practice. In other words, you're not going to get as much bang for your buck from them as you will your more "meat and potatoes" techniques.
Let's take the finger-tip eye gouge for example. This is distinct from shoving your thumb or finger into someone's eye while, for example, grappling (that is, in fact, a very high-yield technique). It is a jabbing strike with a spear-hand at punching range. While it is dangerous to do this to a training partner, it is actually significantly more difficult than punching to the face, because of the precision required to hit the smaller target of the eye, and because of the body's natural flinch response when foreign objects near the eye.
When you fight, you will also suffer a loss of fine motor skills, due to adrenaline dump, that many of these "too dangerous" techniques rely on. So many of the "hit this nerve to cause unconsciousness" styles find that their techniques do not work when pressure-tested. It is surprising how hard it is to hit someone who knows it's coming and doesn't want to be hit. This is true for basic punches and kicks, and doubly so for precision pressure points.
When it comes down to it, people who practice "too dangerous" techniques are basing their beliefs on tradition. A certain amount of tradition is common sense (much of it is BS). There wouldn't be a spearhand in Karate, for example, if a spearhand never worked for anybody at anytime. It is also logical that shoving your finger in someone's eye will hurt, or at least distract them. Nonetheless, these are difficult to pressure-test and therefore should not be go-to techniques.
How To Practice
But your question pertains to how to practice them, not whether or not it is advisable to practice them. Invest in a Century B.O.B. (Body Opponent Bag). It has all the heft of a standing kick bag with human-shaped targets for you to practice your accuracy.
Other than this, you can, as suggested in the other answer, practice your techniques at less than full force or at targets other than the intended ones. For example, while a claw to the groin will leave you with no more partners willing to train with you, you can use a claw to the tricep to assist you in a hip toss.
When in doubt, follow the 80-20 rule, a rule of thumb which, when applied to martial arts, states "You will use about 20% of the techniques your martial art contains about 80% of the time, while you will use 80% of the techniques of your martial art about 20% of the time". The trick is, you should spend about 80% of your time on those 20% of high yield techniques.