Function as a service (like AWS Lambda, Google Cloud Functions and Azure Functions) for Ruby. None of these natively support Ruby ^1 ^2. It's mind boggling that this is the case, especially considering how many Rails/Sinatra apps there are!

^1 Sure there is hacks for getting Ruby running on them, but no native support

^2 Yes I know about Ironworker, from iron.io, but they're going a dockerized and up market and don't even display pricing any more. :(