DeltaChat. I don’t use it myself because it’s built on electron (which basically excludes 99% of modern chat clients); but as it’s technically an email client turned into a chat client, we can assume you’re protected by PGP when writing to most users, and with the added effect of not needing to convince anyone to install anything since from their end it’s just an email.
what’s wrong with glass, metal, and wood? the way i see it, the only reason we changed to plastic is because as long as we have oil wars, the cost for plastic will be lower. go, capitalism. and all that.