If “Send to” menu item disappeared

Copy %userprofile%\AppData\Roaming\Microsoft\Windows\SendTo from the user’s profile that hasn’t disappeared.
Or restore Send To folder default contents:

Open install.wim from “sources” folder of install .iso
Copy 3\Users\Default\AppData\Roaming\Microsoft\Windows\SendTo\ to %userprofile%\AppData\Roaming\Microsoft\Windows\SendTo of user thar has Send To item disappeared.