Apple has removed its Activation Lock status checker on iCloud.com. The removal, which was first discovered by MacRumors, means that if anybody now visits icloud.com/activationlock you'll be greeted by Apple's 404 error page instead of the tool.
The Activation Lock tool allowed potential iPhone or iPad users to check up on whether a device they were about to purchase secondhand had officially had its Activation Lock turned off and was ready for a new user. This was achieved by entering the device's serial number into the Activation Lock status checker, which was able to verify whether the phone was locked or not.
If the device being sold hadn't had its Activation Lock disabled, then not only could a new user not use the device, but it indicated that it was likely stolen.
Whether or not Apple intends to replace the Activation Lock status checker tool remains to be seen. For the meanwhile, though, it seems wise to ask for as much information as possible about a secondhand device when acting as a buyer going forward.