Hm, I'm not sure why it says that. Homebrew does not support any of the plugins in their formula/api/repo.
Here's an open ticket about the plugins on Certbot. There was an old workaround, and there is a new workaround. Depending on what version of homebrew/formula you have, one should work:
Also
Adding:
I just found that second ticket and will rephrase @_az's advice presented in it:
You should not use homebrew, and instead install certbot (and all plugins) from pip.