You are encountering the Activation Lock for iPhone, iPad, and iPod touch - Apple Support feature or its related service iCloud - Find My - Apple . Activation Lock is designed to prevent anyone else from using your iPhone, iPad, or iPod touch if it's ever lost or stolen. Deactivating either of these requires knowing the Apple ID and password that set them, or placing a special request to Apple to reset the device.
In your case with an original receipt in your name Apple can completely reset a device if a person can provide sufficient evidence of valid ownership. This is done online at: Activation Lock support request - https://al-support.apple.com/#/additional-support Apple says, "Proof of ownership documentation is required," but does not provide details in the article as to what is acceptable proof. Prior to this official service Apple typically required the original receipt* issued by Apple or an Apple Authorized Reseller and documented provenance (so typically it would be the original owner who does this, or a descendant with legal proof of ownership).
*Apple recommends you keep your receipt after you buy an Apple device. --> Keep your receipt after you buy an Apple device - Apple Support