Yes, it is true that most of the Martial Art are developed for fighting in a war. But as time passes the methods of fighting a war are changed. In ancient times, the war happens with bows and arrows but at this modern age, we were using guns and technologies to fight.
But still, whenever our bullets are empty or our technology is destroyed by the enemies we have only left the knife and our bare hands to fight. That means still the Martial Art may be useful for use in some cases even in this modern warfare.
So as your question is which Martial Art is useful in this modern age. The answer is that every Martial Art is useful for fighting. You should learn any kind of Martial Art it will be helpful for you in fighting because in fighting your willpower and the mentality is the main thing.
Yes, the techniques depend on the fighting but firstly your willpower and the mentality is important.
Your willpower should be very strong and it should be like never back down. And your mentality should be like you should never fear of your opponent. After that only, Martial Art depends on fighting.
If you want to choose then choice by studying the history of Martial Art how it was formed.
For example, Ninjas were the ancient warriors with great achievements in the warfare. So here you may learn lots of war tactics that were used in ancient times. Also, there is available other Martial Art.
If you want to be the best warrior than my suggestion is that learn every kind of Martial Art you can and try to create your own style. Because while trying to create your own style you will give whole your mind on fighting and it may be the reason for you to become the best fighter.