The Class Diagram item is missing in Visual Studio 2019.
For some reason this is not installed by default in Visual Studio 2019, so we simply need to install it.
1. In Visual Studio click Tools > Get Tools and Features…
2. Close Visual Studio.
3. In Visual Studio Installer:
- Click Individual components
- Search for Class Designer
- Select Class Designer
- Click Modify
4. Wait for it to finish installing, and then launch Visual Studio.
Now you’ll be able to add Class Diagram items again.