Sheikah Slate Health Sensor
The Sheikah Slate Health Sensor was a function of the Sheikah Slate in Breath of the Wild that did not make it to the final form of the game. It served to display the health bar of enemies in a similar manner as the Champion's Tunic.
First shown in E3 2016, the Health Sensor required the use of the Scope function to point at enemies while a timer eventually displayed their health bar.
Removal From The Game
The Sensor relied on motion controls of subjects that can be notoriously mobile and fleeting like Keese, which made it a cumbersome feature to control, as it had to focus on the subject for the Sensor's time gauge to be completed.
Aside from that, the difficulty levels of enemies were already hinted at by their skin colors, progressing naturally through red, blue, black, silver and gold.
The usage of the Sensor, as it worked through the Slate's Scope feature, removed the player's awareness of their surroundings, making them vulnerable to attacks.
This function was removed in favor of making it a feature of wearing the Champion's Tunic, which performs the same function without the extra steps it would have entailed.