It's unclear which formulas are added only as build dependencies. As a result, it's difficult to understand which dependencies can be removed (saving disk space) without damaging other formulas.
brew list --installed-on-request
brew leaves --installed-as-dependency
brew deps --tree --installed
The --annotate flag does not work with --tree.
Can help if you want to remove formulas / casks, but fair to break other formulas.
% ./scripts/brew_deps.py -h
usage: brew_deps.py [-h] [-t TARGET | -v | -q] [formula ...]
produces brew-tree.json which includes all the build/runtime/test ... dependencies
json can be quickly viewed using jg or a similar tool