Custom Rubocop

.rubocop.yml

require:
  - ./lib/rubocop/cop/lint_rails_env

Lint/RailsEnv:
  Enabled: true
  Exclude:
    - 'config/**/*.rb'

lint_rails_env.rb

module RuboCop
  module Cop
    module Lint
      class RailsEnv < RuboCop::Cop::Cop
        MSG = "Avoid Rails.env in application code, " \
              "use configuration parameters instead"

        def_node_matcher :rails_env?, <<~PATTERN
          (send {(const nil? :Rails) (const (cbase)
            :Rails)} :env)
        PATTERN

        def on_send(node)
          return unless rails_env?(node)

          add_offense(
           node.parent.type == :send ? node.parent : node,
           location: :selector, message: MSG
          )
        end
      end
    end
  end
end