[xenorplxx]

You can definitely check out https://github.blog/2018-09-28-upgrading-github-from-rails-3... and other stories of big Rails upgrade to get a grasp of what has changed. As for new features, I'd just go with googling what's new in Rails 4/5.

[xenorplxx]

Oh, I've just noticed there's similar blog post on GitLab's blog now: https://about.gitlab.com/2019/05/28/upgrade-to-rails5/