Shurikens will seek out and pop Bloons automatically.
~ BTD6 description
Seeking Shuriken is the first upgrade of Path 3 for the Ninja Monkey in BTD6. Shurikens will now aggressively home onto bloons, regardless of where the bloons are. Shurikens generally home at bloons based on the target priority of the Ninja at the time they were shot out.
Unlike BTD5, this upgrade is on a separate path to the Distraction upgrade.
Seeking Shuriken enhances the shurikens by allowing them to always follow bloons, maximizing the pierce potential of each shuriken. Adding it with Path 1 upgrades will increase the overall potential of shuriken attacks, with 2-0-1 Ninja acting as a quite solid early-game option. Seeking Shuriken is especially useful as the crosspath for Upper Path 1 upgrades (Double Shot, Bloonjitsu, and Grandmaster Ninja), as this will maximize the DPS of its more powerful shurikens.
This upgrade greatly increases the coverage and power of the Ninja by allowing its shurikens to maximise their pierce usage.
It is recommended to get Sharp Shurikens for this because then the seeking shurikens can maximize its maximum 4 pierce per shot.
Seeking Shuriken did not receive any direct balance changes since it was first released in BTD6, although it is now mutually exclusive with Distraction when interacting with Top Path Ninja.
The only worthy mention is the "Camo Prioritization" target priority that was given to all Ninjas in general. Strategies involving seeking for camos may be positively affected.
As ninja is the 'Camo Specialist' and has been since inception, it is the only tower that will unlock Camo Prioritization on the base tower. [...]
Crosspathing in BTD6 (Top: Path 1, Bottom: Path 2)
The BTD6 Seeking Shuriken upgrade icon features a blue shuriken, but the shurikens in-game remain the same color. All Insta-Monkey upgrades along Ninja Monkey's Path 3 feature blue shurikens, implying that a visual change to the shurikens was initially planned but scrapped.