How to add BlockComment extension for Xcode 11?
I found this link to app support at the bottom of the app store page you linked. That should be a go-to place always.

https://github.com/superk589/BlockComment


  from mac App Store
  
  
  Run the application at lease once, then close it.
  Go to System Preferences -> Extensions -> Xcode Source Editor and enable this extension
  Reopen Xcode and you can find this extension from Xcode menu.