in ruby or rails
This isn't a life-threatening warning, but annoying...In my case, it occurred becouse the gemfile referred to an older version af a installed gem. To remove this message, add to your ~/.bashrc:
export RUBYOPT='-W:no-deprecated -W:no-experimental'
But be aware, the best way is to find the gem with #rubyforge_project specified in its gemspec, and update this gem...
Take a second look at your terminal, and look for a warning like these:
Gem::Specification#rubyforge_project= called from /Users/rolf/.rvm/gems /ruby-2.5.5/specifications/archive-zip-0.12.0.gemspec:16.
This means that you have to remove this specific version, perhaps, more, depending on the amount of warnings - for this gem, or any other gems. In my case, a lot...
For example, let's go with 'archive-zip'.
gem uninstall archive-zip
It will remove the gem, prompt to select a version, or you can remove all. Gem install archive-zip will install the latest version. This solution takes more time, but I think simply disabling warnings isn't something we should go for...
happy days ;-)