Is this new? I swear I searched as recently as last week and the only thing that came up was "rmtree" and the "brew bundle dump && brew bundle --force cleanup" workarounds.

It’s not “last month new” for sure. It was in the list of commands in the manpage last week.

brew might have a discoverability challenge. I've been installing an rmtree helper to do this for years and didn't even know it was an option to autoremove them.