I think it has an plugin on github but it doesn't works more.. Search more of this, perphaps you can found it. PHP: https://github.com/PresentKim/ThrowPocketMinePlugins/tree/299a9ad53f01c02e0f942619f70ab9f04f167090/G/AntiBlockLauncher%20v1.0
no, unless the luncher author choose to embed a marker i doubt it so dont even think about asking for hacked clients to tell the server they are hacked
the plugin you mentioned checks if the login packet has been sent twice. I doubt that that still works, but it looks like it was a way to detect it at some time.
That plugin was using the error of the block launcher at the time. There seems to be no right way to distinguish block launcher (And very dirty source code ...)
The issue is because blocklauncher is a game wrapper, not the game itself. There is no special identifier. you may be able to identify it via an exploit within the wrapper, but not otherwise.