As per enterprise guidelines, any .NET code we checked in must have the corresponding unit test code written with 100% code coverage.
We have .NET code implemented for Templating, Event Handlers, Workflow Implementation, Customization, Reporting, etc.
We started with the Templating code and found that every code file uses the Engine object (Tridion.ContentManager.Templating.Engine) in the implementation. Since Engine is an abstract class, we cannot instantiate it directly in our unit test code because of this we created a wrapper class that inherits from Engine. However, this is not allowed because the Engine class contains an internal abstract method named CheckInitialized().
Here we refer the Engine class as one example only but the concern applies to other Tridion Classes also (if any).

We need SDL to address this issue and make these Tridion classes compatible with the creation of unit test code, which is now an enterprise-wide standard.
Please add the required fix in the next Tridion Sites Release 10.2.0