Starting with Windows 10 build 18237, Microsoft added an acrylic blur effect to the sign-in screen background. The translucent texture of this transient surface helps you focus on the sign-in task by moving the actionable controls up in the visual hierarchy while maintaining their accessibility.
Starting with Windows 10 build 18298, users could turn on or off all transparency effects that would also turn on or off the acrylic blur effect on their sign-in screen background.
Starting with Windows 10 build 18312, Microsoft added a new group policy setting allowing you (administrator) to enable or disable the acrylic blur effect feature on the sign-in screen background for all users without affecting their transparency effects setting.
If you enable the acrylic blur effect on the sign-in screen background, users will still have to turn on transparency effects for their account to see it.
This tutorial will show you how to enable or disable the acrylic blur effect on the sign-in screen background for all users in Windows 10.