Cornerstone websites are gone.
I didn't look too far into things, but there are definitely very large changes since I was "into" Ruby, back in mid-2006.
Rubyforge was lost in 2014 ∞
Announced today that we're sunsetting RubyForge. Last day will be May 15th. Begin your data migration now!
Rubygarden was lost in 2007 ∞
RubyMentor was lost ∞
rubymentor.io was aquired by Devschool (which I think is https://devschool.rocks/)
RubyTalk was lost in 2006 ∞
It seems to have been moved to https://web.archive.org/web/20190831170216/http://blade.nagaokaut.ac.jp:80/ruby/ruby-talk/index.shtml
.. it's strange for it to be gone since http://rubycentral.org/ is still around.