Slightly off topic, but the page says "we don't want your email" and I vaguely remember a site that was looking up public keys on GitHub and getting the email address from the GitHub account. Not sure if I got that right, or what the site name was, but it's definitely possible to get public keys from GitHub users like so: https://github.com/binwiederhier.keys
If you mine these, along with crawling GitHub for email addresses, you can map public keys to email addresses.
Best practice and what everyone should be doing is have different ssh key for every different server, that is, create a new ssh key for prose.sh with a custom file name.
Of course there will be many people reusing the same key for everything and I think it's obvious that doing that increases the risk exposure
If you mine these, along with crawling GitHub for email addresses, you can map public keys to email addresses.