Free Software Webmail Systems (fsf recommended email providers)
"Sign-up works without non-free JavaScript" is a super weird criteria for selecting an email platform.
You would hate LibreJS.
I'm still trying to understand what "proprietary JavaScript" means.
Disroot stopped using RainLoop long time ago when people became aware of a security bug in RainLoop and the fact that the RainLoop project appeared to be dormant. I think Disroot switched to SnappyMail, and then to Roundcube.