Our internal Discourse has happily been running the same old version for a good long while now, but we are finally getting rid of IE9 so it’s way overdue for an upgrade.
I can report that this guide is still good with the latest versions of Discourse. Even with RHEL8 instead of 7 
While experimenting I figured out a way to get those discourse-prometheus and discourse-calendar plugins working offline too even with the extra dependencies, the trick was to fish out the ruby gems from the plugin directory of the build server as well as the ones in /var/www/discourse/vendor/bundle/ruby
docker run -it -v ~/local/rubygems.org:/local-rubygems local_discourse/app /bin/bash -c "cp -rv /var/www/discourse/vendor/bundle/ruby /local-rubygems"
docker run -it -v ~/local/rubygems.org/plugin-gems/discourse-calendar:/local-rubygems local_discourse/app /bin/bash -c "cp -rv /var/www/discourse/plugins/discourse-calendar/gems /local-rubygems"
docker run -it -v ~/local/rubygems.org/plugin-gems/discourse-prometheus:/local-rubygems local_discourse/app /bin/bash -c "cp -rv /var/www/discourse/plugins/discourse-prometheus/gems /local-rubygems"
and then in /templates/web.template.yml :
- exec:
cd: $home
hook: bundle_exec
cmd:
# copy local ruby cache
- cp -rv /local-rubygems.org/ruby/* $home/vendor/bundle/ruby/
- cp -rv /local-rubygems.org/plugin-gems/* $home/plugins/
- su discourse -c 'bundle install --local --deployment --retry 3 --jobs 4 --verbose --without test development'
By the way it looks like the topic for Active Directory IIS SSO got lost at some point, but the code is still available at GitHub - laktak/discourse-sso: Single Sign On for Discourse with Active Directory and still works with the now renamed DiscourseConnect SSO