List all Gentoo packages built against the old ruby or specific version

After a similar article about Python (List all Gentoo packages built against the old python or specific version) under Gentoo, the Ruby upgrade in Gentoo may have similar problems, so this article shows how to list old Ruby modules and packages build against old ruby environments. Despite a clean upgrade without any blockers or masked packages, sometimes, on old machines, there might be packages left still built against an old version of Ruby, which is no longer available on the system!

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equery with USE ruby_targets_ruby26

On Gentoo, there is one important flag in the configuration make.conf file:

RUBY_TARGETS="ruby30 ruby31"

The RUBY_TARGETS controls the support of multiple Ruby versions installed on the system. When the Gentoo package has a use flag ruby, the builder emerge will build the Ruby module for all the Ruby versions in RUBY_TARGETS.
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