Knees and feet are not soft tissue, but they can be targeted in a fight.
However, there are some things to consider.
Typically, at least in the West, people who fight in the street do not target either feet or knees. They tend to head-hunt or try to kick the groin or both. I do not think I have ever seen someone intentionally try to take out a knee or an ankle in a 'real' fight, and in the highly-simulated world of point sparring, such attacks are typically disallowed. I'm not saying it could not happen, I'm saying you just don't typically see it.
Feet are generally protected by shoes in a real self-defense type situation. Ankles may be vulnerable and may in fact be injured, but in my experience, it's usually done by the fighter to themselves, stepping wrong and rolling or twisting the ankle, falling or being knocked down and landing on something, etc. In other words, collateral damage, not intentional damage.
The knee is a real target, from the correct angle and with the right attack, that's for sure. But it is seldom targeted in a 'real' type of fight. I don't know why, but that's how it is. One thing is that knee damage is very difficult to recover from. If someone punches you in the face and you get a broken nose or a black eye, you'll heal. Bust the knee and you have some real issues and won't be back to normal for quite some time and will probably need serious surgical intervention.
What I am trying to say is that the knee is a good and legitimate target in a self-defense situation, but I just haven't seen it used much, either in attack or defense.
It is also a bit harder to train knee attacks, as you can't really do it full power on a resisting opponent. Hard to build up the skills needed.