That said, I think a lot of the value of Firefox Send, which I used a bunch, was that it was maintained by a trusted brand in Mozilla. Indie groups won't have that reputation. And if I wanted to self-host, I would just use magic-wormhole.
Mozilla's instance was obviously associated with the big names 'Mozilla' and 'Firefox' which made it easy to abuse. That isn't an issue now. I guess that helps.
> I wonder if there is a solution that would correctly deal with both situations (never requiring the other end to sign up or know anything about encryption).
I'm actually in the midst of building out a tool for that exact scenario[0]. You can send it to anyone so long as you know the email or username they use for a service that implements OAuth. If they successfully authenticate and the server sees their username / email match up, it sends back the encrypted data to the frontend, which then decrypts it with the key in the URI fragment. It's neither audited nor open source just yet, so I don't recommend using it for actual secret sharing until then.
https://twitter.com/ryanmcgrath/status/1368027469068963843/p...
There's also a nice go port (single binary) of it: https://github.com/schollz/croc
For communications in the other direction, such as when you want to make sure the other person sends you the content securely, there is https://github.com/whitesmith/hawkpost (disclaimer, this was an hackathon project I participated some years ago). Hawkpost doesn't even store the encrypted content.
I wonder if there is a solution that would correctly deal with both situations (never requiring the other end to sign up or know anything about encryption).