Earlier quoted context omitted.
You are correct that Screen Hero still works and all, but it is technically "dead." No one new can sign up for the service. You can only use it if you already had it setup. I would personally consider that "dead" if it's now closed off for anyone to signup with. It's not like you can recommend this as alternative to what Slack is doing with their screen sharing. On https://screenhero.com/login they even say "New sign…
I typed /hero in slack It gave a link to "add the Screenhero Integration" I clicked that link and it took me to a Slack prompted me to create a "free account", which it did using my Slack information. Then I was able to download screenhero. So, anyone new can sign up for the service via Slack.
You do not have a Screenhero account for your Slack email