I would have built this if it didn't already exist: Ente Paste. It offers end-to-end encrypted secret sharing, which expires immediately after the first view, and doesn't require any setup, making it easy to use with non-technical contacts.
For a while I've been uncomfortable sharing PII via normal messaging channels where it can sit in history or, worse, be exposed unencrypted. Ente Paste gives me a very simple way to share that data that resolves most of the major risks.
Great idea Ente team, thanks for building it!