How to disable Windows 10 defender service (W10 1909)

The correct action is to disable the Tamper protection in Windows security/virus&threat protection settings. Even without the key HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows Defender\DisableAntiSpyware = 1 the group policy Turn off Windows defender antivirus to enabled will have effect.

Registry keys should be used as last option.