Apple Activation Lock. Apple stops this webpage, they don’t give any specific reason why.
Apple has stop Activation Lock Status tool that will make it easier for people buying a used iPhone, iPad, or iPod touch to avoid getting a device that is locked to another user.
Accessible via iCloud.com, the Activation Lock Status Checker allows users to enter a Device IMEI or Serial number to check whether a device has Activation Lock turned on. Apple Activation Lock
Activation Lock was introduced alongside iOS 7 and is designed to prevent iPhones and iPads from being stolen. When Find My iPhone is enabled, it effectively locks an iOS device to a user’s Apple ID account, and even when wiped, the device will require the original Apple ID and password.
Activation Lock has cut down on iPhone-related thefts in major cities, but it has also affected users who purchase an iOS device used. If Activation Lock is enabled, a used iOS device will be entirely useless until unlocked by the original owner. Apple Activation Lock
In alternative to check iPhone iCloud Check (Activation Lock) using imei or serial try this Link: https://myicloud.info/icloud-check/
If an iOS device does have Activation Lock enabled, Apple’s tool will give users a clear warning that an Apple ID and password will be required before another user can activate the device. It also provides instructions on how to remove Activation Lock from a used device, which requires contacting the previous owner.