Relevant thread:
https://forum.unity.com/threads/script-halo-has-the-same-name-as-built-in-unity-component.629008/#post-5027504

The overloads of the AddComponent and GetComponent methods which take in a string parameter with the Type name have already been deprecated and in fact calls to them automamatically get replaced in the source code (see my comments in the submitted example project, Case 1324045).

Further the Generic Constraint "where T : Component" has already been added to the AddComponent<T> method.

This means that the 2 remaining steps to fix this issue are:

1. Add the Generic Constraint "where T : Component" to remaining methods GetComponent<T>, GetComponent(s)InChildren<T>, GetComponent(s)InParent<T>, etc. This will prevent being able to even attempt to call them with a custom class as Generic Parameter (e.g. "GetComponent<Grid>();") if this custom class derives from System.Object, as this would then throw a proper Compiler Error instead.

2. At the place where the Warning is thrown add a simple check for "typeof(T).NameSpace" and only proceed to throw the Warning if there is no namespace. To make the Warning message clearer, change it to state that in this case there is ambiguity with the built-in Unity component and that the user should fix this by adding a custom namespace.

And that should be enough to fix this issue.

This is a serious and ridiculous bug. How come you cannot use a name such as "Terrain" when developing a video game.

C# already has a feature to support this scenario, it's called namespaces. In case Unity's developers are not familiar with, let me refer them to the documentation:
https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/namespaces/