I like to use MIST (macOS Installer Super Tool) to grab old macOS versions: https://github.com/ninxsoft/Mist
Apple also provides instructions for downloading many older macOS versions via your terminal: https://support.apple.com/en-us/102662#terminal
Nice, thanks for sharing! It'd be interesting to integrate MIST into lume's ipsw command - right now Apple's native features in Apple Vz only provides download links for the latest supported version of the host, so grabbing older versions requires workarounds like this.