init.rb

module RedmineDefaultPrivateComments
  module LabelledFormBuilderPatch
    def check_box(field, options = {}, checked_value = "1", unchecked_value = "0")
      if @object_name.to_s == "issue" && field.to_s == "private_notes"
        options = options.symbolize_keys
        issue = @template.instance_variable_get :@issue
        options[:checked] = issue&.private_notes_or_default
      end
      super(field, options.except(:label), checked_value, unchecked_value)
    end
  end

  module IssueJournalPatch
    def self.prepended(base)
      base.class_eval do
        after_initialize :set_private_notes_default
      end
    end

    def set_private_notes_default
      if new_record? && self.respond_to?(:user) && self.respond_to?(:project)
        self.private_notes = user.allowed_to?(:set_notes_private, project)
      end
      super if defined?(super)
    end

    def private_notes_or_default
      @current_journal&.private_notes || User.current.allowed_to?(:set_notes_private, project)
    end
  end

  module Patches
    def self.apply
      Issue.prepend IssueJournalPatch
      Journal.prepend IssueJournalPatch
      Redmine::Views::LabelledFormBuilder.prepend LabelledFormBuilderPatch
    end
  end
end

class AfterPluginsLoadedHook < Redmine::Hook::Listener
  def after_plugins_loaded(_context = {})
    RedmineDefaultPrivateComments::Patches.apply
  end
end

Redmine::Plugin.register :redmine_default_private_comments do
  name "Redmine Default Private Comments"
  author "Intera GmbH"
  description "This plugin makes comments private by default."
  version "0.1.0"
  url "https://github.com/Intera/redmine_default_private_comments"
  requires_redmine :version_or_higher => "6.0.0"
end