Read this text! It will answer all of your questions.
This is the official method to detect whether a project is running in a compiler like TurboWarp or forkphorus. This block won't make your project automatically run faster or unlock new features. It can be used to automatically enable laggy features in compilers without impacting the normal Scratch site or to suggest users to visit a link for a better experience, but you should always allow users to use the project in Scratch anyways, even if it's really slow.
https://turbowarp.org/414716080
https://forkphorus.github.io/#414716080
How it works:
This is not a hacked block or a hidden block. This is a custom boolean argument reporter. You can create one with the "add input (boolean)" button when editing a custom block. Normally these blocks always report 0 when used outside of a custom block, but there's some code in TurboWarp and forkphorus that looks for this block and makes it return true instead.
Other Scratch compilers are welcome to implement this. Thanks to
@Executec for the idea of using boolean arguments.