I genuinely think someone from the 2.x days could read a "how to migrate to Rails 3" guide, skim some release notes for each of the subsequent major releases, and be generally up to speed. I've been working with Rails almost continuously since the 3.x days, and if you ignore some of the more library-like additions like ActiveStorage or Credentials, things are structurally extremely similar between 3.2 and 6.0. The biggest differences to a casual reader would probably be Ruby syntax itself developing from Ruby 1.9 forwards to this point. Things were quite different in the 1.8.x days.
Models live in the same spot. Controllers live in the same spot. Your classes generally still inherit from the same stuff, albeit with some additions to the tree like for ApplicationRecord in places where you might previously have inherited directly from ActiveRecord::Base.
The stability is one of my favorite parts of Rails. Its still modern and very usable, but I don't need to relearn it all the time.
IMO it really has not changed much even from the 1.x days it’s just a bit more polished with AR having where syntax now instead of only find and better solutions for organizing code that isn’t strictly model/view/controller. But if i opened a rails 6 or rails 1, it’d be pretty easy to find my way around...
Avoid too many after hooks and before filters(er actions) and the code will be easier to follow and maintain
Models live in the same spot. Controllers live in the same spot. Your classes generally still inherit from the same stuff, albeit with some additions to the tree like for ApplicationRecord in places where you might previously have inherited directly from ActiveRecord::Base.
The stability is one of my favorite parts of Rails. Its still modern and very usable, but I don't need to relearn it all the time.