Menlo::Legacy is a package to install Menlo::CLI::Compat which is a compatibility library that implements the classic version of cpanminus internals and behavios. This is so that existing users of cpanm and API clients such as Carton, Carmel and App::cpm) can rely on the stable features and specific behaviors of cpanm.
This way Menlo can evolve and be refactored without the fear of breaking any downstream clients, including 'cpanm' itself.