I released RubyGems 3.0 in last year. RubyGems 3 is bundled with Ruby 2.6 that is the latest stable version of the Ruby language. You can use it easily today.
The new version of RubyGems has many of improvements, bug fixes and some incompatibilities. I put them into the changelog that is https://blog.rubygems.org/2018/12/19/3.0.0-released.html. But its changelog was generated from github pull-request. So it's not human-readable article.
I will describe the details of RubyGems 3.0 changes and the future plan of RubyGems 4.0.