The convention programmers have long followed is now enshrined as something called Semantic Versioning (which comes with its own version, lol). Basically if a library makes an incompatible change it's supposed to increment the major version. So going from Rails 4.x to Rails 5.0 is a sign that you can't upgrade. Which is great in theory, except that the tools we programmers use to manage versions don't understand SemVer and just pick the latest version by default. Which causes programmers to explicitly ask for the version they want to avoid moving to the next. And all the 'pinning' in turn causes libraries to be less than disciplined about incrementing the major version number when they break something. So it's all a mess.