Hello
Thank you for posting in Microsoft Community forum.
The feature you're referring to was part of the Mixed Reality Viewer app in Windows 10, which allowed users to place 3D models into their real-world environment using the computer's camera. This app was later rebranded as 3D Viewer.
To use this feature now, follow these steps:
Open the 3D Viewer app: If it's not already installed, you can download it from the Microsoft Store.
Load a 3D model: You can use pre-existing models or create your own in Paint 3D.
Activate Mixed Reality mode: Within the 3D Viewer app, select the "Mixed reality" button. This will turn on your computer's camera and allow you to place the 3D model in your real environment.
If you don't see the app, it might not be installed on your system. You can download it from the Microsoft Store.
Best Regards,
Wesley Li