I am familiar with Python ... and I can understand that FastCGI was never really supported in the first place there, especially since Python has WSGI.

I am not as familiar with Ruby.

But just because "modern apps and frameworks" don't recommend it doesn't mean it is a bad protocol. PHP with PHP-FPM is awesome and well supported. Fast CGI still has a place in this world, and I don't see why just because some apps/frameworks have decided it isn't worth their time to support it, it would be an invalid choice to make for a new app...