A Rake task appears in rake -T if it has a description:

desc 'Compile assets'
task :compile do
  ...
end

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To not list it, simply omit the description:

task :compile do
  ...
end

You can also hide a Rake task that has been defined by someone else (like a gem) by removing the description:

Rake::Task['compile'].clear_comments

Or you can whitelist which tasks should be listed:

visible_tasks = %w(compile build package)
Rake::Task.tasks.each do |task|
  visible_tasks.include?(task.name) or task.clear_comments
end