if @simon's answer is not working in some of the mac's please follow the below process.
If you have already installed swiftgen using the following commands:
$ brew update
$ brew install swiftgen
then follow the steps below in order to run swiftgen with older version.
Step 1: brew uninstall swiftgen
Step 2: Navigate to: https://github.com/SwiftGen/SwiftGen/releases
and download the swiftgen with version: swiftgen-4.2.0.zip.
Unzip the package in any of the directories.
Step 3: Execute the following in a terminal:
$ mkdir -p ~/dependencies/swiftgen
$ cp -R ~/<your_directory_name>/swiftgen-4.2.0/ ~/dependencies/swiftgen
$ cd /usr/local/bin
$ ln -s ~/dependencies/swiftgen/bin/swiftgen swiftgen
$ mkdir ~/Library/Application\ Support/SwiftGen
$ ln -s ~/dependencies/swiftgen/templates/ ~/Library/Application\ Support/SwiftGen/
$ swiftgen --version
You should get: SwiftGen v0.0 (Stencil v0.8.0, StencilSwiftKit v1.0.0, SwiftGenKit v1.0.1)