This page in the docs https://middlemanapp.com/basics/build-and-deploy recommends middleman-deploy but the latest version of middleman breaks support. You can work around it with gem ‘middleman-deploy’, ‘~> 2.0.0.pre.alpha’ but the docs should be modified to help people get around the issue or just not recommend the extension and provide an alternative.
For anyone stumbling across this, I had a few issues+warnings when using gem middleman-deploy, ~> 2.0.0.pre.alpha, so I’ve created a fork with a few extra commits cherrypicked from the original repo.
This works for me in a Middleman 4.3.6 install:
gem 'middleman-deploy', git: 'https://github.com/cabgfx/middleman-deploy', branch: 'master'
Have only tried deploying with FTP, but that worked flawlessly.
Also, one of those commits included in my fork specifically addresses an rsync issue, so I believe that option would work as well, but haven’t tried it myself.
HTH