We have a potential fix to allow -e to still be used: create an empty file in /etc/logstash/conf.d. Call it whatever you like, e.g.
sudo touch /etc/logstash/conf.d/empty.conf
Even though the file is empty, it should prevent this error:
Should this work for those trying to use -e to do the test config in the docs, we will put an empty config file in /etc/logstash/conf.d in future package releases.
It's right here in the breaking changes documentation.