Microsoft has removed the ability to disable Microsoft Defender and third-party security software via the Registry to prevent malware from tampering with protection settings. Malware developers have known about the DisableAntiSpyware group policy, and malware developers have as well. Microsoft also stated that if a user removes their installed antivirus solution, Windows Defender will automatically turn back on to protect them. The impact of the registry removal is limited to Windows 10 versions prior to 1903 using Microsoft Defender Antivirus. This change does not impact third party antivirus connections to the Windows Security app.
Source: https://www.bleepingcomputer.com/news/microsoft/malware-can-no-longer-disable-microsoft-defender-via-the-registry/