Naruto Shippuden brought a lot of new abilities, characters and weapons into the world. Old characters were introduced with brand new looks and skills. One of the characters that was reinvented was Sasuke Uchiha. When Sasuke made his introduction in Shippuden, it was a memorable one. He was much stronger than the last time Naruto saw him and he carried a new sword called Kusanagi.
Sasuke's Kusanagi was much different than the original Kusanagi wielded by Orochimaru. The version used by Orochimaru could be controlled mentally and extend its blade a great distance. The Kusanagi wielded by Sasuke lacked all the abilities that made Orochimaru's sword unique. Besides being able to channel lightning chakra through the blade, Sasuke's Kusanagi seemed like a regular sword. However, it did have one unique ability.
The Lost Power of Kusanagi
The first time that Sasuke uses his sword is against the new Team Kakashi when they are trying to capture him. He attempts to slash Sakura with his sword but Yamato intercepts the attack. When Yamato tries to block the attack with his kunai, Sasuke's sword phases through the blade and pierces into him. Sasuke then explains that his Kusanagi is impossible to block.
After this one interaction, this incredible ability attached to Sasuke's sword is never mentioned again. Sasuke goes on to fight many different opponents with his sword, all of whom block and parry his strikes. There is no explanation given as to why they're able to block Kusanagi or why the sword's ability no longer works. Kusanagi's inability to be blocked is forgotten and Sasuke carries a regular sword for the rest of the series.
Was Kusanagi Just Too Powerful?
Although a canon explanation as to why Sasuke's sword lost its special ability is never given, the true reason might be as simple as this: it was too powerful. A sword that's impossible to block would be far more powerful than almost any other weapon in the world of Naruto. The weapons carried by the Seven Swordsmen of the Mist would pale in comparison to a sword that cannot be defended against. Any person not familiar with Sasuke's weapon would be immediately at a severe disadvantage.
Obito possessed a similar ability to Sasuke's sword. With his Sharingan, Obito could slip through attacks and make it so no one could touch him. He was only defeated by Kakashi, who had the same ability and high speed, who could attack him before he could activate his power. Being able to phase through the attacks and defense of your opponent is an almost unbeatable ability. It's possible that Kishimoto decided that Sasuke already had too many powerful skills and removed that one from his sword.
Sasuke's Sword Needed Something More
With Kusanagi no longer being impossible to block, Sasuke just had a regular sword for the rest of the series. Although Sasuke displayed impressive skills with the weapon, there was no longer anything about it that made it special. He could channel his lightning chakra through the sword, but that was a skill almost everyone possessed in Naruto. Kakashi, Asuma, Killer Bee and all the samurai from the Land of Iron could channel their chakra through their weapons to increase their power.
Sasuke's sword was a key part of his fighting style and an iconic part of the series as well. It deserved to have at least one special ability to set it apart from all the other basic weapons in the show. When Sasuke loses the sword during Fourth Shinobi War, there was no impact because the audience knew that he could simply pick up any other sword and be just as effective.
In fact, he does just that. In Boruto, Sasuke has a brand new sword that also lacks any unique powers. Through a flashforward, it's revealed that Sasuke's sword eventually gets passed down to Boruto. This transition of wielders would have a much greater impact if the sword itself was special in some way. As it is now, it's just a normal sword that Sasuke uses and will be used by Boruto.