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