Nothing new here. Sites like privnote.com have been around much longer.
PGP, end-to-end encryption is everywhere now.
How do you prevent the URL from becoming something which needs to be kept secret?
For example, here's a secret I just put into Retriever. Are you able to see it? https://retriever.corgea.io/#eyJhbGciOiJSU0EtT0FFUC0yNTYiLCJ...
And as I understand it, there's no way to verify you're talking to the right person, so sharing a secret via signal is strictly better?