It is unclear why Microsoft made the decision to change the behavior of Microsoft Defender in this regard. You can turn the program’s real-time protection off temporarily, but if it’s off for a while Windows will turn it back on automatically.

Many people are looking for ways to disable or remove it from their system as they prefer to use other software, for example on Windows 8 and 10 you cannot turn off Microsoft Defender completely anymore, A click on Settings in Microsoft Defender opens the control panel for the program in the new Windows Settings application. The problem with all Defender versions in Windows Vista and above is it’s integrated into the operating system and installs by default with no visible or hidden option to uninstall. Some spyware protection is better than none, and it’s built in and free! But… if you are already running something that provides great anti-malware protection, defender will probably wasting precious resources, there’s no need to have more than one application running at a time. It’s a small piece of software that runs in the background to help protect ,your computer from malware (malicious software) like viruses, spyware, and other potentially unwanted software.

Since Vista,Microsoft Defender is included with Windows.

Enabling windows defender