Here's some info for those that need to use a workaround on OSX.
1. Download the old launcher and copy it to your Applications folder. (You can rename it something like MinecraftOld.app and keep the new launcher as well.)
2. To run the old launcher on OSX, you need the old version of Java, 1.7. To avoid having to uninstall newer versions of Java, you can temporarily switch versions by opening Terminal and running:
export JAVA_HOME=$(/usr/libexec/java_home -v 1.7)
Then you can run the launcher from the terminal with:
java -d64 -jar /Applications/MinecraftOld.app/Contents/Resources/Java/Bootstrap.jar
Hope this helps somebody!
Confirmed 19w36a