Every formula in homebrew-core is open source — Homebrew accepts nothing else — so the split here is permissive versus copyleft, not open versus proprietary. The licence is the SPDX expression each formula declares. Weak copyleft (LGPL, MPL, EPL) counts as copyleft; a dual licence offering a non-copyleft alternative counts as permissive. Grey is formulae Homebrew has since dropped, whose licence the API can no longer report.