We had an older project that we are updating to C#11 and .NET 7.0 and noticed that the nullable compiler warnings were not showing. These are turned on in new projects (C#8.0 and above in VS2022 etc) by default.
But if you load an older project and look in the project properties, you will see the dropdown empty:
Select ‘Enable’ in this setting and the compiler warning will show. For example: