Tamper Protection blocks attempts to modify Windows Defender Antivirus settings through the registry. To help ensure that Tamper Protection doesn’t interfere with third-party security products or enterprise installation scripts that modify these settings, go to Windows Security and update security intelligence to version 1.287.60.0 or later. Once you’ve made this update, Tamper Protection will continue to protect your registry settings and will log attempts to modify them without returning errors.
If the Tamper Protection setting is On, you won't be able to turn off the Windows Defender Antivirus service by using the DisableAntiSpyware group policy key.